Yes. Registration with the competent tax authorities is required; however, the registration regimes applicable to direct taxation and to value added tax (“VAT”) operate independently of one another, being administered by distinct authorities pursuant to separate legal frameworks.

For the purposes of direct taxation, taxpayers are registered with the Administration des contributions directes (“ACD“).

In the case of individuals, no separate or formal application for registration is required of a Luxembourg tax resident. Fiscal registration is, as a practical matter, effected automatically upon the individual becoming subject to tax.

In the case of companies, registration is linked to the commencement of business activity. A company is required to be registered with the ACD to satisfy its tax obligations. In this regard, the filing of the company’s articles of association with the Luxembourg Trade and Companies Register (Registre de commerce et des sociétés) automatically triggers the issuance of a national identification number (matricule national). As a matter of practice, the company is thereafter contacted directly by the competent tax office (bureau d’imposition). In the absence of such contact, it remains incumbent upon the company to approach the ACD of its own initiative. The competent corporate tax office is responsible for matters of registration, assessment and collection in respect of direct tax liabilities.

VAT is administered separately, by the Administration de l’enregistrement, des domaines et de la TVA (“AED“). A taxable person subject to a registration requirement must declare the commencement of its economic activity to the AED, further to which a VAT identification number is allocated.

Luxembourg VAT legislation expressly requires any taxable person established in Luxembourg to notify the AED of the commencement, any subsequent modification and the cessation of its taxable activity.

It follows that registration for corporate income tax purposes does not, in itself, constitute or satisfy the requirement for VAT registration, and conversely, registration for VAT purposes does not extend to, or substitute for, registration for corporate income tax purposes. The two registration regimes are legally and administratively distinct, and must each be complied with separately.

For direct taxes, the tax liability is formally determined by a tax assessment issued by the tax authorities, not by pure self-assessment, even though the taxpayer’s tax return plays a central role in the process.

Taxpayers, whether individuals or corporate entities, must file an annual tax return. The tax return serves as the material basis for the tax assessment: the competent tax office relies on the data declared by the taxpayer to fix the tax liability. However, the taxpayer’s declaration does not enjoy any presumption of accuracy, and the ACD is neither bound by it nor required to accept it as correct. The competent tax office retains the power and duty to verify tax returns, to audit taxpayers and to correct, of its own motion, any errors it identifies, including those that lead to a lower tax charge. The ACD will then issue a tax assessment (Steuerbescheid / bulletin d’impôt) on that basis. The taxpayer’s role is therefore declaratory and cooperative (good faith and sincerity), but the ultimate determination of the tax is made by the tax authorities, not by the taxpayer through self-assessment.

The jurisprudence confirms that filing the tax return opens the assessment procedure under §§204–227 of the General Tax Law of 22 May 1931 as amended (Abgabenordnung, “AO”) and that the ACD determines the taxpayer’s liability through the tax assessment.

Importantly, where the tax office intends to make a material departure from the  filed return to the taxpayer’s disadvantage, §205(3) AO requires that, in principle, the tax office gives the taxpayer an opportunity to comment on the intended rectifications before issuing its tax assessment.

VAT operates differently and is much closer to self-assessment: the taxable person files periodic/annual VAT returns. If the AED disagrees, it may issue a bulletin de rectification or, in the statutory circumstances, a taxation d’office.

Yes, but an important distinction must be made between correcting the return before assessment and changing an assessment after it has become final.

With respect to direct taxes, where the tax office does not itself detect an error committed by the taxpayer, the latter may file a rectifying tax return if they become aware of the mistakes. The submission of a rectifying tax return obliges the tax office to determine the tax on the new basis declared (§ 85 AO). A taxpayer may at any time submit corrected information or a rectifying return. However, the filing of a rectifying return after a tax assessment has been issued does not, in itself, reopen or replace the initial assessment.

The filing of a rectifying return is mandatory where the error committed by the taxpayer operates to his benefit (§ 165e(1) AO). This obligation continues to apply even after the issuance of a tax assessment based on the erroneous tax return; in such a case, the tax office issues a corrective assessment on the ground of a “new fact” (§222 AO). The obligation to file a rectifying tax return subsists until the tax claim is time-barred; failure to file a rectifying return in these circumstances constitutes tax fraud or attempted tax fraud.

Once a tax assessment containing an error that is unfavourable to the taxpayer has been issued, the taxpayer can no longer compel the tax office to issue a corrective assessment, its task being deemed completed. Once an assessment exists, the taxpayer must rely on the appropriate procedural remedy, principally a timely claim under § 228 AO, or, where its requirements are satisfied, one of the statutory provisions permitting modification of an assessment (such as § 94 AO). Case law includes instances in which a rectifying return was filed after assessments had been issued and the tax authorities refused to amend them; the ensuing dispute concerned whether the conditions of § 94 AO or § 100a AO permitted modification. A claim against a tax assessment must, as a rule, be filed within three months of notification (§§ 228 and 246 AO). This period constitutes a limitation period (délai de forclusion), after the expiry of which a claim is no longer admissible.

After this deadline, the taxpayer may always request a limited modification of the assessment, confined to the specific item affected by the error they have made, but the tax office has discretionary power to amend the assessment in this limited respect or to maintain it unchanged.

From a Luxembourg tax perspective, the tax authorities may challenge and increase the amount initially assessed by (i) departing from the taxpayer’s declaration at the initial assessment stage, (ii) correcting errors ex officio, (iii) issuing rectifying assessments based on new facts or rectifying returns, and (iv) reassessing following audits or under specific statutory modification provisions.

For the reassessment of direct taxes, the main mechanisms available to the ACD include:

• examination of the return before the original assessment under §§204 et seq. AO;
• requesting information and supporting evidence;
• audits/revisions and investigations;
• assessment by estimation (taxation d’office) under §217 AO, where the tax base cannot reliably be established, notably because the taxpayer has failed to comply with its filing obligations or information requests;
• issuance of assessments subject to review under §100a AO, where applicable;
• amendment of an existing assessment under §222 AO, notably where new facts or evidence subsequently become known, which justify a higher tax assessment;
• specific correction provisions such as §94 AO.

The courts stress that the tax office must seek to establish the taxable base as accurately as possible. Taxation by estimation is essentially a mechanism used where the tax authorities have exhausted the reasonable possibilities of investigation without being able to establish the taxable base accurately.

These measures do not depend on a prior formal “justification” given to the taxpayer; rather, they must comply with the applicable legal rules, and the reasoning must be capable of being explained and defended if the taxpayer lodges a claim or appeal within the statutory time limits.

However, for an amendment made under §222 AO, new facts or evidence capable of justifying the amendment are required. The jurisprudence confirms that a final assessment cannot simply be reopened at will.

In addition, where the tax office materially departs from a filed return, §205(3) AO normally requires that a prior explanation must be provided to the taxpayer; the Court has confirmed that this also applies to a rectifying assessment under §222 AO.

For direct tax purposes, the general limitation period is five years. However, a ten-year period applies in cases of failure to file a return or where an additional assessment results from an incomplete or inaccurate return, irrespective of whether fraud was intended. Article 10 of the Law of 27 November 1933 provides for these periods, which run from 1 January of the year following the year in which the tax claim arose. Accordingly, the ten-year period does not require fraud, and this point is of particular importance.

Case law confirms that § 222 AO may be relied upon where new facts or evidence justify an increased assessment, provided that the tax claim is not time-barred. It also expressly refers to the extension from five to ten years in the relevant situation governed by § 222 AO.

Once prescription has definitively occurred, the tax authorities cannot, as a rule, create an additional tax liability merely by reopening the assessment. That said, the rules on interruption and suspension of prescription remain applicable, and a taxpayer may validly agree to a waiver of prescription (renonciation à la prescription). The ACD describes such a waiver as an act by which the taxpayer renounces the benefit of prescription vis-à-vis the tax authorities.

For VAT purposes, the general limitation period is five years. Pursuant to article 81 of the Luxembourg VAT Law, the right of the Treasury to collect VAT and related penalties, as well as the right to claim restitution of VAT or penalties, is time-barred after five years, calculated from 31 December of the year in which the VAT became due or, in the case of restitution, from 31 December of the year to which the tax to be refunded relates or in which the penalty was imposed.

Unlike the rules applicable to certain direct taxes under article 10 of the Law of 27 November 1933, the statutory limitation period for VAT is not extended to ten years in cases of non-filing or incomplete or inaccurate returns, irrespective of whether fraud is involved. The limitation period for VAT therefore remains five years, subject to the rules on interruption and suspension and to the possibility of a valid waiver of prescription by the taxpayer.

VAT prescription can be interrupted (e.g. by recovery measures or by a waiver of prescription), in which case a new five‑year period starts and is acquired at the end of the fourth year following the last interruptive act.

Luxembourg direct-tax law distinguishes several levels of fraud.

Under §396(1) AO, simple intentional tax fraud (fraude fiscale simple) essentially involves intentionally obtaining an unjustified tax advantage for oneself or another, or intentionally causing tax revenue to be understated.

Fraud requires a material tax effect. The case law emphasises the requirement that the tax reduction or unjustified advantage has actually occurred; for assessment taxes, fraud is consummated when the tax has been assessed too low or an unjustified tax advantage granted or maintained.

There are then two criminal categories:

• Aggravated tax fraud – §396(5) AO: where the tax evaded exceeds one quarter of the tax actually due, subject to a minimum of €10,000, or where the tax evaded exceeds €200,000 (with corresponding rules for undue refunds).
• Tax swindle (escroquerie fiscale) – §396(6) AO: where a significant amount is involved and the fraud is committed through the systematic use of fraudulent manoeuvres intended to conceal relevant facts from the authority or persuade it of incorrect facts.

Luxembourg law also recognises unintentional tax fraud (§402 AO). The administrative courts distinguish it precisely from §396 AO fraud because the reduction of tax or tax advantage was not intentionally sought.

For Luxembourg VAT, the notion of fraud is aligned with the direct‑tax concept but is defined and sanctioned specifically in the VAT Law. In substance, VAT fraud covers any intentional conduct that leads to evading VAT, reducing VAT due, or obtaining an undue VAT refund, with gradations between “simple” and aggravated fraud and a category of escroquerie fiscale (tax swindle).

No, not every case of “tax fraud” leads to a criminal investigation, because Luxembourg deliberately distinguishes administrative tax fraud from the two criminal categories.

Simple tax fraud (§396(1) AO) is dealt with by the tax authorities and is sanctioned by an administrative fine. Simple VAT fraud gives rise to administrative tax penalties, calculated as a percentage of the VAT avoided or unduly reimbursed, subject to statutory minimum amounts.

Aggravated tax fraud (§396(5) AO) and tax swindle (§396(6) AO) are criminal offences and fall within the exclusive jurisdiction of the judicial authorities.

The statutory penalties are:

• aggravated tax fraud: 1 month–3 years’ imprisonment plus a fine from €25,000 up to six times the tax evaded/undue refund;
• tax swindle: 1 month–5 years’ imprisonment plus a fine from €25,000 up to ten times the amount concerned.

The law expressly provides that the judicial authorities alone have jurisdiction over §396(5) and (6) AO.

When the conduct qualifies as aggravated VAT tax fraud, the offender is liable to criminal sanctions, including imprisonment and a fine that may amount to a multiple of the VAT avoided or the undue reimbursement. Furthermore, VAT law punishes tax swindle (escroquerie fiscale) by more severe criminal sanctions, including longer terms of imprisonment, higher fines calculated as a multiple of the VAT avoided or undue reimbursement, and possible ancillary penalties such as partial or total loss of specified civil rights for a determined period.

Luxembourg tax law does not provide for any formal, published code of conduct or detailed procedural manual specifically governing the manner in which the tax authorities are to conduct audits. The applicable framework is instead derived from general principles of administrative and tax law, supplemented by certain scattered procedural rules governing controls and investigations, as set out principally in the Abgabenordnung (“AO“).

The AO imposes certain procedural constraints upon the tax authorities, including obligations relating to the conduct of investigations, rules governing taxpayer cooperation, and the taxpayer’s right to be heard. However, no implementing regulation has been adopted by the Luxembourg executive to further specify the practical modalities of such controls, with the consequence that the Administration des contributions directes (“ACD“) retains what may be characterised as an extensive measure of discretionary authority in this regard. A notable exception is a Grand-Ducal regulation enacted in 1989, which curtailed the tax authorities’ investigatory powers by prohibiting audits in respect of certain banking and financial establishments.

It should further be noted that taxpayers are not provided with any formal guidance or communication from the ACD as to the procedure to be followed in the event of an audit.

Governing principles: necessity and proportionality

Notwithstanding the absence of a codified framework, the exercise of audit powers by the tax authorities is subject to the overarching requirement that any control be conducted only insofar as is necessary and useful. While the investigatory powers of the tax office must be sufficiently robust, and where appropriate incisive, to ensure their effectiveness, such powers must nonetheless be exercised so as to avoid any drift toward arbitrariness or undue harassment of the taxpayer. It follows that the tax authority’s powers may only be exercised with restraint. This requirement derives both from the rules governing the exercise of the tax office’s discretionary authority and from the general principle of proportionality applicable in administrative law.

In practice, this principle operates on three levels. First, the tax office may only have recourse to means that are appropriate to achieving the intended objective (the principle of suitability). Second, among the range of suitable measures available, the tax authorities must select that which least infringes upon the private interests of the taxpayer (the principle of necessity). Third, the effects of the measure ultimately adopted must be weighed against the public interest pursued (the principle of proportionality stricto sensu).

These principles give rise to a hierarchy governing the tax office’s investigatory measures. A tax audit must start with a request for information aimed at clarifying any ambiguous aspects of the taxpayer’s tax return. In the absence of a satisfactory response, the tax authorities will then request the production of the relevant supporting documents, and it is only where the probative value of such documents proves insufficient that the tax authorities will require the taxpayer to provide further justification. The principle of proportionality further entails a subsidiarity requirement with respect to recourse to third parties: the tax authorities must have exhausted the available avenues of cooperation with the taxpayer before addressing themselves to third parties, in accordance with § 209 AO.

Tax secrecy

Finally, the principle of tax secrecy prohibits the disclosure to third parties of information obtained by tax officials in the course of tax proceedings. This principle constitutes the necessary corollary to the extensive investigatory powers conferred upon the tax authorities in the exercise of their functions. Any breach of the obligation of tax secrecy is subject to criminal sanction pursuant to § 412 AO.

Power to compel production of information

Yes. The tax office is vested with a right of information (droit de renseignement), the purpose of which is to resolve any doubt, ambiguity, or risk of confusion in respect of the taxpayer’s affairs. This right extends to all information of fiscal relevance in the taxpayer’s possession. A failure to respond, or the provision of insufficient or dilatory responses, may be sanctioned, including by way of a periodic penalty payment (astreinte) or, in appropriate cases, an assessment made ex officio. Where the right of information concerns transactions entered into with parties related to the taxpayer, the taxpayer’s duty of cooperation is correspondingly reinforced, such transactions being, by their nature, more difficult to verify (§ 171(3) AO).

This right of information is complemented by a right of communication (droit de communication), which constitutes its natural extension and enables the tax office to obtain access to documents held by the taxpayer or by third parties. Such communication may take the form of: (i) automatic communication, as in the case of the annual statement of medical fees transmitted by the social security fund; (ii) spontaneous communication of documents deemed relevant to the taxpayer’s assessment, such as where the AED communicates undeclared turnover to the ACD for purposes of upward revision of taxable income; or (iii) communication requested by the tax office. Where the taxpayer is a trader, such communication will generally concern the records required to be maintained under the Commercial Code, including a detailed breakdown of the balance sheet and profit and loss account (§ 172(1) AO). As a general matter, the right of communication is confined to documents that are relevant for purposes of the taxpayer’s assessment, private documents being of no assistance in clarifying the taxpayer’s fiscal position.

The exercise of the right of communication is warranted where reasonable doubt exists as to the accuracy of the tax return, in which case the tax office may require the production of such evidence as may reasonably be expected of the taxpayer.

Taxpayers subject to business taxation may additionally be subject to an in-depth audit of their accounts, conducted at the taxpayer’s business premises. The tax office may not, however, conduct a visit to the taxpayer’s private residence against the taxpayer’s wishes, save to the extent necessary to make such assessments and valuations as are required for the determination of tax (§ 173 AO), the inviolability of the taxpayer’s domicile being guaranteed under the Constitution.

Under the AO, the tax office is thus vested with broad information-gathering powers, and taxpayers are subject to corresponding duties of cooperation and proof. Pursuant to § 171 AO, a taxpayer must, upon request, substantiate the accuracy of the tax return, clarify doubtful matters, and prove assertions to the extent reasonably to be expected in the circumstances. Documents and business records may accordingly be requested.

Extension to emails

Under Luxembourg tax law, the tax authorities may indeed require the production of emails in the context of an audit; however, this prerogative is not absolute and is subject to a defined legal framework, as follows:

Right of information and communication (§§ 171 and 172 AO): The tax authorities’ rights of information and communication permit them to request any information or document useful to verifying the accuracy of the taxpayer’s return and in the taxpayer’s possession. Business emails—for instance, those recording the terms of a transaction, the justification for an expense, or commercial agreements—fall within the scope of such documents.

Requirement of fiscal relevance: The tax authorities may only require the production of documents that are relevant from a fiscal standpoint. The right of communication is confined to matters useful to the assessment of tax; emails of a strictly personal or private nature need not be produced, as they contribute nothing to the clarification of the taxpayer’s fiscal position.

Principle of proportionality: Any audit must remain within the bounds of what is necessary and useful. The tax authorities may not demand unrestricted access to the entirety of an email account, such conduct being tantamount to an impermissible “fishing expedition.” Requests must be precise and appropriately tailored to resolving a specific doubt or verifying a particular matter.

Limits arising from professional secrecy: Where emails contain communications protected by a privilege enforceable against the tax authorities, such as legal professional privilege attaching to communications with counsel, as recognised in the case law of the CJEU and under Luxembourg law, the taxpayer or third-party holder may lawfully decline to produce them.

Right of appeal

The availability of a right of appeal depends materially upon the legal character of the measure in question. A binding or executory administrative decision may be challenged through the applicable administrative remedies, whereas a mere informal request for information is not, as such, independently subject to appeal.

Under Luxembourg tax law, the available remedies against a request for information vary according to the context in which the request is made—namely, whether it arises in the course of a domestic audit or in the context of international administrative assistance.

Domestic audits: A simple request for information or supporting justification addressed by the tax office to a taxpayer (or to a third party) constitutes a preparatory act in the process leading to the assessment of tax.

• Absence of direct remedy: Being merely preparatory in nature, such a request does not constitute an administrative decision producing adverse legal effects (acte faisant grief). Consequently, no direct contentious remedy may be brought against the request itself.
• Indirect remedy: The taxpayer may nonetheless challenge the relevance or lawfulness of such a request at a later stage, whether by way of an objection (réclamation) against the final tax assessment, or by challenging any sanction imposed for failure to respond (for example, an appeal against a decision imposing a periodic penalty payment or fine).

International administrative assistance: Where the ACD issues a request for information in the form of a formal injunction decision (typically in response to a request from a foreign tax authority), a considerably stricter legal framework applies, particularly following the judgment of the CJEU in Berlioz and the Luxembourg law of 1 March 2019:

• Action for annulment: The holder of the information (i.e., the recipient of the injunction) is entitled to bring an action for annulment before the Administrative Tribunal. It should be noted that the taxpayer under investigation abroad may not bring such action where that taxpayer is not itself the addressee of the injunction.
• Subject matter of the action: The action serves, in particular, to enable judicial verification that the request satisfies the criterion of “foreseeable relevance,” ensuring that the tax authorities have not engaged in an impermissible fishing expedition.
• Time limit and effect: The time limit for bringing such action is shortened to one month from notification of the injunction. The action has suspensive effect on the obligation to transmit the requested information pending the court’s determination.

Power to compel production from third parties

Yes. The Luxembourg tax authorities are vested with the power to compel the production of information from third parties. Where a taxpayer refuses to cooperate, or does not itself possess the information requested, the tax office may address requests to third parties—such as banks, fiduciaries, or employers—who are presumed to hold documents relevant to the matter under enquiry.

Paragraph 175 AO expressly provides that persons who are not themselves parties to the relevant proceedings may be required to furnish information concerning facts relevant to tax supervision or to the determination of tax claims. Information so provided must generally be accurate and complete, and may require the third party to consult available documents and business records.

In the specific context of international administrative assistance (exchange of information upon request), where the requested information is held by a third party, the Director of the Direct Tax Administration (“ACD”) may issue a formal injunction (décision d’injonction) compelling that third party to produce the required information, typically within one month. Failure to comply with such an injunction may give rise to substantial administrative fines of up to €250,000.

Exceptions to the duty of cooperation

Certain exceptions to this duty of cooperation exist, most notably in respect of professional privilege, including the professional secrecy attaching to the legal profession, which is fully enforceable against the tax authorities.

Luxembourg domestic law now recognises, without restriction, the opposability of lawyer-client privilege against the tax authorities, including in respect of tax advice. The previous limitation under § 177(2) AO—which excluded privilege in respect of matters brought to the lawyer’s attention in the context of tax advice, save where the client faced a risk of criminal prosecution—has been abolished by legislative intervention. The courts have accordingly held that lawyer-client privilege is, in principle, opposable to ACD investigations, subject to the specific rules under § 177 AO concerning tax advice and representation; privilege likewise applies where a response would expose the client to criminal prosecution.

Banking secrecy constitutes the other principal exception to the duty of third-party cooperation, being opposable to the tax authorities, including where the latter act in the context of tax advice. This exception is grounded in § 178bis AO, pursuant to which no information for purposes of a taxpayer’s assessment may be sought from credit institutions, save pursuant to an express statutory derogation. This normative choice reflects a deliberate policy consideration closely linked to the attractiveness of the Luxembourg financial centre.

It should be noted, however, that banking secrecy cannot, as a general matter, be invoked to resist a request for information made in the context of international exchange of information.

Right of appeal

Whether a third party enjoys a direct judicial remedy depends upon the legal basis and nature of the information order in question. Particular rules apply, for instance, to information orders issued in the context of international exchange-of-information proceedings.

A specific right of appeal exists against an injunction to produce information. Under the law of 25 November 2014, as amended, the third-party holder of the information is entitled to challenge the injunction before the competent tax judge (the Administrative Tribunal). The principal features of this appeal process are as follows:

• Suspensive effect: The filing of an appeal suspends the transmission of the requested information to the foreign tax authority pending the judge’s decision.
• Scope of review: The tax judge will examine whether the information sought is not “manifestly devoid of any foreseeable relevance” (absence manifeste de pertinence vraisemblable).
• Accelerated procedure: In order to avoid undue delay in international cooperation, the appeal procedure is subject to expedited timeframes. The third party is generally required to file the appeal, together with supporting submissions, within one month of notification of the injunction, and the court is likewise required to render its decision within an accelerated timeframe (typically within one month of service of the last submission filed by the parties).

Where the judge finds the injunction to be unlawful, it will be annulled, together with any administrative fines imposed in connection with non-compliance.

Luxembourg law does not recognise a general equivalent to the US-style “closing agreement”, pursuant to which the tax authority and the taxpayer may freely compromise the amount of tax legally due. Tax liability is determined by strict application of the law, and the tax authorities do not, as a general matter, enjoy discretion to bargain away a statutory tax liability. This principle derives from the constitutional principle of legality in tax matters (principe de légalité de l’impôt), under which tax is a matter of public order (ordre public).

This does not, however, preclude the amicable resolution of disputes arising in the course of an audit. A distinction must be drawn between questions of law and questions of fact.

Questions of law

Any agreement between a taxpayer and the tax office that purports to contravene the tax law, or that bears upon the legal interpretation of applicable rules, is void and without legal effect. The determination of the tax due must, in all cases, rest upon a strict application of the law to the relevant facts, and cannot properly be the subject of negotiation or compromise.

Questions of fact

By contrast, the taxpayer and the competent tax office (bureau d’imposition) may reach a binding arrangement in respect of factual elements of the tax file—for example, the determination of the professional as opposed to the private portion of a mixed expense. The validity of such a factual arrangement is subject to two cumulative conditions:

• there must exist a genuine, objective difficulty in precisely ascertaining the relevant factual element absent the taxpayer’s cooperation; and
• the resulting arrangement must lead to a reasonable determination of the taxable base, the object of the exercise being the most accurate possible determination of the tax liability, rather than a means by which the tax office may avoid the burden of further enquiry.

Where such an agreement is reached, it produces effects analogous to those of a civil settlement (transaction) and becomes binding upon both the tax office and the taxpayer. Consequently, any subsequent challenge to, or litigation concerning, the factually agreed element becomes precluded.

Administrative sanctions

Where the matter at issue concerns administrative sanctions, such as fines or late-payment surcharges, the tax authorities enjoy a measure of discretionary power, and may decide unilaterally not to impose a penalty, or to waive a penalty already pronounced. This should, however, properly be characterised as a unilateral waiver on the part of the tax authorities, rather than a negotiated settlement.

Mandatory administrative pre-litigation phase

Independently of the foregoing, Luxembourg procedure provides for a mandatory administrative claim (réclamation administrative préalable) to be filed with the Director of the ACD prior to the taxpayer’s ability to bring proceedings before the administrative courts. This phase serves as a filter designed to relieve the courts of unnecessary litigation and functions, in practice, as a final avenue of dialogue between the tax authorities and the taxpayer with a view to resolving the dispute amicably and without recourse to judicial proceedings.

International mutual agreement procedure

Finally, in an international context involving double taxation—for instance, following a transfer pricing adjustment identified during an audit—a Mutual Agreement Procedure (“MAP“) may be initiated. This is a formal mechanism whereby the competent authorities of the states concerned negotiate with a view to reaching a binding agreement eliminating the double taxation in question, without recourse to judicial litigation.

Several remedies may be available, depending upon the specific nature of the taxpayer’s grievance and the stage of the proceedings.

Challenge to formal decisions

Where the tax office has issued an executory decision, the taxpayer may raise procedural objections with the tax office or the Director and challenge the decision through the appropriate administrative remedy. Where a tax assessment has been issued, a claim under § 228 AO is available. Where such a claim has remained pending before the Director of the ACD for a period of six months without a decision having been rendered, the taxpayer is entitled, though not obliged, to bring the matter before the Administrative Tribunal.

Concerns arising during an ongoing audit

Where the taxpayer’s concern relates to the manner in which an audit is being conducted, or its duration, prior to the issuance of any challengeable decision, the available remedies are less clearly delineated. Luxembourg law does not provide a specific statutory remedy of general application designed to accelerate the conduct of an audit. Nonetheless, several structural safeguards exist:

• Protection against arbitrary treatment: The tax authorities’ powers of control must be exercised with moderation and in accordance with the principle of proportionality. The tax office is required to avoid any drift towards arbitrariness or harassment (chicanerie) with respect to the taxpayer, and its control activities must be confined strictly to what is necessary and useful.
• Duration of the audit and the statute of limitations: It is recognised that an in-depth audit (contrôle approfondi) may, by its nature, extend over several weeks or even several months. The principal legal constraint upon the duration of an audit is the statute of limitations: for direct taxes, the tax authorities generally have five years within which to establish the tax due. Should the tax authorities allow this period to lapse through undue delay, theyforfeit their right to collect the tax in question.
• Hierarchical recourse: A taxpayer who considers that they are being treated unfairly, or subjected to unreasonable delay, may escalate the matter through the administrative hierarchy. The Director of the ACD, as hierarchical head of all administrative personnel, is empowered under § 46 AO to intervene and to direct a tax office to act in a specified manner.
• The right to be heard: The taxpayer is further protected by the adversarial principle (principe du contradictoire). At the conclusion of an in-depth audit, the auditors may not issue a tax assessment by way of surprise; they are required to communicate to the taxpayer a draft report (projet de rapport) setting out any criticisms or proposed adjustments, thereby affording the taxpayer a genuine opportunity to state its position and mount a defence prior to the issuance of any final report or tax assessment.

State liability

Independently of the foregoing, a taxpayer may seek to hold the State liable in damages where the tax authorities have committed a fault. A culpable delay (retard fautif) on the part of the Director of the ACD, or an abnormal delay in the conduct of proceedings, may constitute such a fault, giving rise to the civil liability of the State. To succeed in such a claim, the taxpayer must establish both that the tax authorities failed to observe the requisite standard of conduct, and that such failure resulted in quantifiable financial loss.

Yes. A taxpayer has a formal right of appeal if they disagree with a tax assessment (bulletin d’imposition) for direct tax or VAT purposes. The claim must generally be filed within three months of notification of the tax assessment.

To validly exercise these rights of appeal, the taxpayer must have legal capacity (capacité d’agir) and a direct, personal, and current interest in the matter (intérêt à agir). This means the assessment must negatively affect their specific tax situation (for example, a taxpayer cannot generally appeal a zero-tax assessment).

Both, in sequence.

The appeal process is highly structured and must follow specific mandatory stages.

From a direct tax point of view:

1. Mandatory administrative claim (Réclamation administrative préalable): Before going to court, the taxpayer must first file a formal administrative complaint with the Director of the Direct Tax Administration (“ACD”).

• • Deadline: This claim must be filed within three months from the date the tax assessment was notified to the taxpayer.
  • Requirement: This step is compulsory. If a taxpayer bypasses the Director and goes straight to court, the judge will declare the case inadmissible.

• • Scope: The Director conducts a full reconsideration of the tax assessment. The jurisprudence states that a valid claim triggers an integral ex officio re-examination and the Director is not limited by the taxpayer’s arguments.

2. First-instance judicial appeal (Tribunal administratif): If the taxpayer disagrees with the Director’s decision regarding their administrative claim, they can escalate the matter to the judicial level by filing a lawsuit before the Administrative Tribunal.

• • Deadline: The appeal must be filed within three months of notification of the Director’s decision.
  • Silence of the Director: If the Director fails to respond to the initial administrative claim within six months, the taxpayer is legally entitled to bring the case directly to the Administrative Tribunal without waiting further.
  • Scope: For direct taxes, the Tribunal generally acts as a “judge of the merits” (juge du fond in a contentieux de réformation), meaning it has the power to thoroughly review the facts and the law, and can actually recalculate and modify the tax assessment, not just cancel it.

3. Second-Instance Judicial Appeal (Administrative Court): If the taxpayer (or the State) is unsatisfied with the judgment rendered by the Administrative Tribunal, they have the right to appeal to the Administrative Court (Cour administrative).

• • Deadline: This appeal must be lodged within 40 days of the notification of the Tribunal’s judgment.
  • Representation: For this stage, the taxpayer is legally required to be represented by a lawyer (avocat à la Cour).

Administrative Courts’ decisions cannot be appealed before the Cour de cassation (Supreme Court).

From a VAT point of view:

The appeal process for VAT (Taxe sur la Valeur Ajoutée – TVA) differs significantly from that of direct taxes, primarily because VAT disputes fall under the jurisdiction of the civil courts (tribunaux civils) rather than the administrative courts.

However, the general structure of the process still requires a mandatory administrative phase before you can go to court. Here are the specific rules for VAT appeals:

1. Mandatory Administrative Claim (Réclamation administrative): Before taking the matter to court, the taxpayer must file an administrative claim. Bypassing this step will result in the court declaring the case inadmissible.

• • Deadline: The claim must be filed within a strict deadline of three months.
  • Strict motivation requirement: The claim must be “duly motivated” (dûment motivée). This means the taxpayer must provide detailed explanations and submit supporting documents. Failure to provide these justifications can lead to the claim being declared inadmissible.
  • Review process: If the local tax office (bureau d’imposition) does not grant the taxpayer’s request, the file is automatically forwarded to the Director of the AED (the indirect tax authorities). The Director will review the entire assessment and issue a decision (décision directoriale).

2. Judicial Appeal before Civil Courts (Recours contentieux): If the taxpayer disagrees with the Director’s decision, they can take the case to the civil courts.

• • Procedure: The appeal must be initiated via a writ of summons (assignation) served to the AED (specifically to its Director) by an authorized person, such as the company manager or a receiver.
  • Deadline: This summons must be served within three months of the date the Director’s decision was notified to the taxpayer.
  • In case of administrative silence: If the Director fails to issue a decision within six months of the initial administrative claim, the taxpayer does not have to wait indefinitely. They can initiate the lawsuit before the civil courts against the initial tax assessment. In this scenario of silence, no deadline runs against the taxpayer to file the lawsuit.

3. Special Rules for Administrative Fines (Amendes administratives): If the dispute specifically concerns an administrative fine (rather than the VAT assessment itself), the process is slightly different):

• • The initial claim must be sent directly to the Director within three months.
  • The Director has six months to respond.
  • If the Director rejects the claim, the taxpayer has three months to take the case to the Civil Courts. If the Director remains silent for six months, the taxpayer can take the case to court without any time limitation.

Civil Courts’ decisions can be appealed beforethe Cour de cassation (Supreme Court).

The administrative claim (réclamation) submitted before the Director does not constitute a public hearing, and the taxpayer’s tax file is not publicly accessible at this stage of the procedure.

At the judicial stage, by contrast, hearings are, in principle, conducted publicly. The Luxembourg Justice portal publishes decisions of the Administrative Courts and the Civil Courts, including decisions rendered in tax matters, in anonymised form. Such published jurisprudence routinely records that the hearing was held, and judgment pronounced, in public session.

Information identifying the taxpayer is generally anonymised in published judgments, having regard to considerations of tax secrecy and the protection of privacy.

The administrative phase is primarily written.

At the judicial stage, it is predominantly written, supplemented by an oral hearing.

The pleadings and evidence are submitted through written applications/memorials; the case is then heard at a public hearing at which the parties may make oral submissions. Oral arguments play a strictly subsidiary role. Published judgments routinely record the report of the judge-rapporteur and the parties’ oral pleadings.

• Exchange of written memorials: The core of the procedure consists of an exchange of written documents called memorials (mémoires). Parties must present all their arguments and evidence in writing.
• Limitation on oral arguments: The tax judge will only consider the written elements present in the case file. While a public hearing with oral pleadings (plaidoiries) does take place, these oral observations have very little legal weight.
• No new oral arguments: During the oral pleadings, lawyers can only elaborate on what has already been submitted in writing; they cannot add anything new. Any new argument raised for the first time orally at the hearing will be declared inadmissible (with the rare exception of arguments of “public order,” which can be raised at any time).

Not in the common-law sense.

There is no general US/UK-style discovery process requiring each party to disclose all relevant documents, including adverse ones.

The proceedings instead operate through the production and exchange of the documents relied upon by the parties, the administrative file, and the court’s powers to order appropriate evidentiary/investigative measures.

There is a strict process for the exchange of documents, driven by the legal principle of the adversarial process (principe du contradictoire).

• Automatic deposit of the tax file: Once a lawsuit is initiated, the tax authority is required to automatically deposit a copy of the taxpayer’s complete tax file (dossier fiscal) at the court registry (greffe). The taxpayer then has the right to inspect this file on-site.
• Consequences of withholding documents: If the tax authorities fail to submit the file, this is considered a refusal to collaborate and a violation of the taxpayer’s defense rights, which can lead the judge to outright cancel the disputed tax assessment.
• Exchange of evidence: Both the taxpayer and the State (represented by the government delegate) must deposit the specific documents (pièces) they intend to use to support their written memorials. These must generally be submitted before the judge-rapporteur reads their report at the public hearing.
• Limits on discovery (tax secrecy): While the taxpayer has the right to access the documents the tax authorities used to make their decision, this right is limited by “tax secrecy” (secret fiscal). If the tax file contains information regarding third-party taxpayers (e.g., in transfer pricing or comparative audits), the tax authorities must protect their anonymity. In such cases, the communication is often restricted to aggregated averages rather than specific third-party documents to preserve confidentiality.

Such recourse to witness evidence is possible, though uncommon in the ordinary course of tax litigation, primarily by reason of the essentially written nature of the procedure. The procedure before the tax courts is, in principle, documentary in character. Nevertheless, evidence by way of witness testimony may be proposed by the parties, and ordered by the court, where relevant to the matters in dispute. The jurisprudence records instances in which taxpayers have expressly offered proof by witness testimony in respect of contested factual circumstances.

However, there are two specific situations related to third-party evidence or testimony:

1. Written testimonials for procedural issues (proving a mandate): The document explicitly mentions the use of testimony (témoignage or attestation testimoniale) in a very specific procedural context: proving that a representative (mandatary) had the proper authorization to act on behalf of the taxpayer.

• • If a mandate was given orally and its existence is contested, the taxpayer must provide a “testimonial attestation” proving it existed beforehand.
  • Similarly, an illegible signature on a mandate can be validated by “way of testimony”.

2. Court-appointed experts (expertise): Rather than calling lay witnesses to establish facts, the tax judge relies on experts when technical or factual clarification is needed.

• • If the judge needs an opinion on complex factual matters (e.g., the value of real estate, the profit margins of similar businesses, or the verification of accounting figures), they will issue an arrêt interlocutoire (interlocutory judgment) to appoint an expert.
  • This expert is usually a professional, such as a chartered accountant or a specialized lawyer, who conducts an investigation and submits a written report to the court.

In summary, you will not generally see a trial phase where witnesses are called to the stand to be examined and cross-examined about the facts of the tax dispute. The procedure relies on written documents, the exchange of written memorials, and, when necessary, formal written reports from court-appointed experts.

Under Luxembourg tax law, the burden of proof on an appeal against a tax assessment does not rest entirely on the taxpayer; rather, it is generally divided between the taxpayer and the tax authorities depending on the nature of the claim. However, in certain specific situations, the burden of proof falls squarely on the taxpayer.

1. The general rule (divided burden of proof): According to the general principles of the burden of proof, the burden is allocated based on who benefits from the claim:

• • The tax authorities bear the burden of proving facts that trigger or increase the tax liability (e.g., proving the existence of undeclared income or a taxable event).
  • The taxpayer bears the burden of proving facts that reduce or eliminate their tax liability (e.g., qualifying for a tax exemption, or proving that specific expenses are deductible).

The jurisprudence expressly confirms that allocation.

2 Appeals against an ex officio assessment (taxation d’office): If the taxpayer is appealing an ex officio assessment (an estimated assessment issued because the taxpayer failed to file a return, kept irregular accounts, or refused to cooperate), the burden of proof shifts heavily to the taxpayer. Before the tax judge (both in the first instance and on appeal), the taxpayer must prove that the actual figures deviate significantly from the estimated figures used by the tax authorities. The judge will not overturn the tax authorities’ estimate unless the taxpayer can successfully discharge this burden.

There is no reliable statutory “usual duration” for ordinary tax litigation that would run from the initial assessment to the final appeal. However, strict statutory deadlines are provided for each stage of the procedure, which gives an idea of the minimum timeline:

1. Administrative phase (Director of the ACD):

• • Filing: The taxpayer has 3 months to file a claim after being notified of the tax assessment.
  • Decision: The Director has minimum 6 months to make a decision. If no decision is made within this timeframe (administrative silence), the taxpayer has the right to escalate the case to the Administrative Tribunal.

2. Judicial phase – First Instance (Administrative Tribunal):

• • Filing: The taxpayer has 3 months to file a lawsuit after receiving the Director’s decision. (No deadline applies if escalating due to the Director’s 6-month silence).
  • Written Phase:
    • The State’s representative has 3 months to file a response.
    • The taxpayer has 1 month to file a reply.
    • The State has 1 month to file a rejoinder.
  • Judgment: The text does not specify how long the judges take to hold the hearing and issue a judgment, but it notes that the court is constitutionally obligated to rule within a “reasonable time,” acknowledging that the slowness of justice is a known issue.

3. Judicial phase – Appeal (Administrative Court):

• • Filing: An appeal must be lodged within 40 days of the first-instance judgment’s notification.
  • Written Phase: The respondent has 1 month to reply, followed by 1 month for the appellant’s reply, and 1 month for the final rejoinder.

All judicial deadlines for exchanging briefs are suspended during the summer judicial vacation from July 16 to September 15, which can further extend the total duration of the process. Judicial proceedings can then take materially longer and the duration depends heavily on the complexity, written exchanges, evidentiary issues and appeal.

In summary, while the strict procedural steps (filing and exchanging briefs) alone take over a year, the total duration from assessment to a final appeal judgment heavily depends on how long the tax authorities and the courts take to deliberate and issue their respective decisions.

In principle, yes.

Yes, the taxpayer generally must pay the tax assessment pending the outcome of the appeal.

Here are the specific legal principles governing this rule, along with its exceptions:

1. The general rule – no suspensive effect: Filing an administrative claim (réclamation) or a judicial appeal (recours contentieux) does not have a suspensive effect (effet suspensif) on the obligation to pay the tax. The tax remains due by the deadline specified on the tax assessment bulletin. Pay first, contest later. This rule is rooted in the presumption of legality of administrative acts and the necessity to avoid paralyzing the collection of public revenues through procedural delays. If the taxpayer does not pay, the public treasury can initiate forced recovery procedures.
2. Exceptions and mitigation – stay of execution (sursis à exécution): To mitigate the strictness of this rule, a taxpayer can request a stay of execution, which temporarily suspends the obligation to pay. This can be sought at two different stages:

• • During the Administrative Phase: The tax office (bureau d’imposition) has the discretionary power to grant a stay of execution (sometimes even on its own initiative) if the immediate collection of the tax would be inequitable. If granted, this halts the collection and suspends the statute of limitations for the debt.
  • During the Judicial Phase: If the case has moved to the Administrative Tribunal, the taxpayer can file a specific request for a stay of execution before the President of the Tribunal through summary proceedings (procédure de référé).

To succeed, the taxpayer must cumulatively prove two strict conditions:

• • A serious legal ground (moyen sérieux): The argument against the tax assessment must be glaringly obvious upon initial review of the file, making it highly probable that the assessment will ultimately be annulled or reformed.
  • Grave and definitive prejudice (préjudice grave et définitif): Immediate execution of the payment must threaten to cause irreversible harm, such as rendering the taxpayer insolvent.

With respect to direct tax litigation, there are indeed strict rules regarding who may represent the taxpayer, and these restrictions become tighter as the appeal progresses through the different stages of the procedure:

1. Administrative phase (before the Director of the ACD): At this stage, there are very few restrictions:

• • The taxpayer can act on their own behalf.
  • If they choose to be represented, the representative can be anyone with legal capacity (for example, a spouse, though it is usually a legal or financial professional).
  • The representative must have a special and express mandate (a specific power of attorney) to act on behalf of the taxpayer for the precise tax dispute.

2. Judicial phase: First Instance (Before the Administrative Tribunal): Before the lower administrative court, the rules become stricter:

• • The taxpayer is still allowed to represent themselves.
  • If the taxpayer chooses to be represented, the representative must belong to a specific circle of authorized professionals:
    • An Avocat à la Cour (Attorney, List I or V).
    • An Expert-comptable (Chartered accountant, but not a simple accountant).
    • A Réviseur d’entreprises (Statutory auditor).
  • Foreign equivalents (like a German Steuerberater) or individuals with just a law degree (but who are not admitted as an avocat) are not permitted to represent the taxpayer.

3. Judicial phase: Appeal (before the Administrative Court): At the highest level of appeal, the restrictions are absolute:

• • The taxpayer can no longer represent themselves and representation by an Avocat à la Cour is mandatory.
  • Other professionals, such as chartered accountants (experts-comptables) or statutory auditors (réviseurs d’entreprises), are entirely excluded from representing the taxpayer or submitting oral explanations before the Administrative Court.

With respect to indirect tax litigation (VAT), here is who may represent the taxable person at each stage of VAT litigation:

1. Administrative phase:

• • The taxable person may lodge its “Claim” (Réclamation) in its own name.
  • If the taxpayer chooses to be represented, the representative must belong to a specific circle of authorized professionals:
    • An Avocat à la Cour (Attorney, List I or V).
    • An Expert-comptable (Chartered accountant, but not a simple accountant).
    • A Réviseur d’entreprises (Statutory auditor).

2. Judicial phase (First Instance and Appeal): under general Luxembourg civil procedure, representation by an avocat à la Cour (attorney) is strictly mandatory for proceedings before the civil courts and the Court of Cassation. Other tax professionals, such as accountants, cannot represent clients in these civil courts. Where a lawyer is instructed by the taxable person and lodges an appeal, supported by reasons of fact and law, such an act falls within the exercise of the legal profession. In such a case, the company must be represented by a lawyer registered with one of the Bar Associations provided for under the amended law of 10 August 1991 on the legal profession. The provisions governing the exercise of the legal profession are matters of public policy (ordre public), as they are enforced by criminal penalties.

Partially, but not a full English-style “loser pays all legal fees” system.

There is a system regarding costs, but it distinguishes between court fees and professional/legal costs. The shifting of professional costs is not automatic.

Here are the specific rules that apply:

1. Court fees (Frais judiciaires or frais et dépens): There is an automatic cost-shifting rule for official court fees. The judgment will determine which party must bear these costs. If the taxpayer’s appeal is inadmissible or unjustified, the taxpayer pays; if the taxpayer wins, the State pays. However, the document notes that these official court fees are of a “negligible amount.”
2. Legal and professional costs (lawyer fees, etc.): For the actual costs of representation (such as hiring an avocat or other professionals), the general principle is that the taxpayer must bear their own professional costs, even if they win the case.
3. The exception: procedural indemnity (indemnité de procédure): Despite the general rule, a cost-shifting mechanism does exist for professional costs, but it must be specifically requested and is subject to the judge’s discretion.

• For the taxpayer: The taxpayer can request a procedural indemnity. The tax judge may award this if they consider it “inequitable” (inéquitable) to leave the taxpayer to bear the financial burden of the costs they incurred to defend themselves.
• For the State: This rule goes both ways. The State can also be awarded a procedural indemnity to be paid by the taxpayer. This can happen, for example, if the taxpayer requests that the case be struck from the roll shortly before pleadings, or if the taxpayer created a considerable and unnecessary workload for the government delegate defending the State.

For a purely domestic dispute over the amount of tax legally due, there is no general statutory voluntary mediation or binding arbitration mechanism equivalent to commercial alternative forms of dispute resolution (“ADR”).

The administrative objection procedure itself provides an opportunity to resolve the matter without litigation.

International cases are different: applicable double-tax treaties may provide for a Mutual Agreement Procedure (“MAP”) and, depending on the treaty/EU mechanism concerned, potentially arbitration or other dispute-resolution mechanisms.

Yes, as explained above, there is a right of onward appeal in Luxembourg direct tax litigation and for VAT litigation. The system is organized as a “double degree of jurisdiction” (double degré de juridiction), meaning there are two levels of judicial review.

With respect to direct tax litigation, here are the specific levels of appeal:

1. First Instance: The Administrative Tribunal (Tribunal administratif). After exhausting the mandatory administrative phase (the claim before the Director of the ACD), the taxpayer’s first level of judicial recourse is to file a lawsuit before the Administrative Tribunal. This court acts as the trial court, ruling on both the facts and the law.
2. Appellate level: The Administrative Court (Cour administrative). If a party (either the taxpayer or the State) is not satisfied with the judgment of the Administrative Tribunal, they have the right to file an onward appeal (appel) to the Administrative Court. This appeal must be filed within a strict deadline of 40 days after the first-instance judgment is notified.

The Administrative Court has full jurisdiction (effet dévolutif), meaning it reviews the entire case, both on the facts and the law, and substitutes its own decision for that of the lower court.

Is there a higher Supreme Court / Court of Cassation? No. The document explicitly states that the Administrative Court is the highest and final court for direct tax matters. Its decisions cannot be appealed to a Court of Cassation (pourvoi en cassation). Such an appeal is not possible—and deemed unnecessary—because the Administrative Court is a single, unified court with only one chamber.

Therefore, the Administrative Court is the highest appellate court in the jurisdiction for direct tax disputes.

With respect to indirect tax litigation (VAT), here are the specific levels of appeal:

1. First Instance: After the mandatory administrative claim before the Director of the AED (Registration Duties, Estates and VAT Authority), the judicial phase is initiated via a summons (assignation) before the Civil Courts (tribunaux civils)
2. Appellate level (Cour d’Appel): The decisions of the first-instance civil court can be appealed to the Court of Appeal.
3. Supreme Court or Cour of Cassation (Cour de cassation): Unlike the administrative system (which stops at the Administrative Court), the civil system allows for a final appeal to the Supreme Court / Court of Cassation. The Cour de cassation does not re-examine the facts of the case but strictly ensures that the law was correctly applied by the lower courts.

There is no single penalty automatically attached to every additional assessment.

Important sanctions include:

• Late/non-filing surcharge (§168 AO): up to 10% of the tax assessed.
• Simple intentional tax fraud (§396(1) AO): administrative fine between 10% and 50% of the tax evaded or undue refund.
• Unintentional tax fraud (§402 AO): administrative fine between 5% and 25% of the tax evaded/undue refund.
• Aggravated tax fraud: imprisonment 1 month–3 years plus fine of €25,000 to six times the tax evaded/undue refund.
• Tax swindle: imprisonment 1 month–5 years plus fine of €25,000 to ten times the amount.

There are also astreintes, penalties for specific failures to comply, and default interest, which should be distinguished from penalties.

Circular L.G. – A n°67 issued by the ACD on  28 July 2021 , addresses the uniform treatment of §§ 166(3), 396(1) and 402(1) AO concerning incomplete or inaccurate tax returns, simple tax fraud and unintentional tax fraud, respectively, and  provides guidance on the determination of administrative fines.

General framework

The setting of an administrative fine constitutes a discretionary decision sui generis, subject to challenge by way of a claim under § 228 AO. No fixed general rule or specific exception governs the determination of the fine, as such determination is, by its nature, always a matter of discretion. Nevertheless, this discretion is not unfettered: it must be exercised within the statutory minimum and maximum thresholds prescribed for each category of infringement, and every fine imposed must be justified by reference to the particular circumstances of the case.

Factors to be taken into account

In exercising their discretion, tax offices must undertake an effective and explicit assessment of the particular circumstances relevant to fairness and appropriateness. In principle, the fine must be set so as to be:

• Adapted to the circumstances of the case;
• Proportionate to the infringement established; and
• Proportionate to the taxpayer’s ability to pay (capacité contributive).

The tax office must take into account all elements within its possession at the time the decision is made. Relevant considerations include, in particular:

• Whether the taxpayer acted with intent (required for penalties under §§ 166(3) and 396(1) AO) or merely negligently (§ 402(1) AO), this distinction directly affects both the applicable legal basis and the severity of sanction;
• The overall factual circumstances, assessed by reference to the conduct expected of a normally diligent taxpayer;
• The amount of tax evaded or the refund improperly obtained, since the fine is calculated as a percentage of this amount, within statutory bands, namely 5–25% under § 166(3) AO, 10–50% under § 396(1) AO, and 5–25% under § 402(1) AO;
• Whether the tax has, in fact, been assessed, and whether the assessment was preceded by an in-depth audit, an on-site inspection, or a review by the audit department, as this may affect the availability of the § 166(3) AO penalty;
• Where relevant, the involvement of representatives (Vertreter), such as tax advisers acting as intermediaries between the taxpayer and the tax authorities, who may themselves be subject to sanction under § 402 AO.

Procedural safeguards

The circular emphasises the need for uniformity in the practice of all tax offices, so as to avoid disparities in the treatment of similarly situated taxpayers. The decision imposing the fine must be notified to the taxpayer, and such notification triggers both the one-month period for payment and the period within which the taxpayer may exercise remedies. Notification by registered letter is deemed to take effect on the third day following dispatch. The decision must expressly indicate the availability of a claim under § 228 AO as the applicable remedy.

Transfer pricing and anti-abuse provisions are the areas most likely to give rise to challenge by the Luxembourg tax authorities over the coming twelve months, for the following reasons.

1. Transfer pricing

Transfer pricing remains at the forefront of international tax enforcement priorities, driven by the OECD’s BEPS initiative, the EU’s continued focus on aggressive tax planning, and increased exchange of information between tax authorities (including through Country-by-Country Reporting). Luxembourg, as a jurisdiction hosting a significant number of multinational group financing, holding and intellectual property structures, is particularly exposed to this trend.

The ACD has progressively strengthened its audit capabilities and specialised transfer pricing expertise, and this is likely to translate into an increased volume of adjustments and disputes. The ACD established a dedicated Transfer Pricing Competence Centre on 1 October 2025, expressly to assist tax offices and litigation/legal divisions where significant related-party transactions are detected. The ACD states that the centre is intended to develop a more coordinated approach to increasingly complex transfer-pricing audits and also supports multilateral audits.

Intra-group loans, cash-pooling arrangements, and the remuneration of intangible assets are traditionally difficult to benchmark by reference to comparable arm’s-length transactions, making them a natural focus for adjustment. Given the prevalence of financing and IP-holding structures within Luxembourg corporate groups, these arrangements present a heightened risk of challenge regarding the arm’s-length nature of pricing, particularly where documentation is insufficiently robust to withstand audit scrutiny.

2. Substance requirements and anti-abuse rules

The general anti-abuse rule under Luxembourg tax law, reinforced by the EU Anti-Tax Avoidance Directive (“ATAD”) provisions transposed into domestic law, has become an increasingly prominent tool for challenging arrangements considered to lack genuine economic substance. The evolving jurisprudence of the CJEU (notably the “Danish cases” on beneficial ownership and abuse in the context of the Parent-Subsidiary and Interest-Royalties Directives) has encouraged tax authorities across the EU, including Luxembourg, to scrutinise more closely whether entities benefiting from tax treaty or directive relief possess sufficient substance and are not interposed for the principal purpose of obtaining a tax advantage.

3. Interaction between transfer pricing and anti-abuse analysis

These two areas are increasingly intertwined in practice: a transfer pricing adjustment may itself trigger, or be accompanied by, an anti-abuse challenge where the tax authorities take the view that the pricing or structuring in question was designed principally to secure a tax advantage rather than to reflect a genuine commercial rationale. This convergence increases the overall likelihood, and potential severity, of challenges in this area, as an adjustment may result not only in a re-characterisation of the taxable base but also in the denial of treaty or directive benefits altogether.

In my opinion, two areas in particular present significant difficulty for taxpayers confronted with a challenge by the Luxembourg tax authorities: the mastery of tax procedure, and the practical burden of proof.

1. Procedural rigour and the risk of dismissal on formal grounds

A notable proportion of cases brought before the administrative courts do not result in a decision on the merits of the underlying tax dispute, but are instead dismissed on procedural grounds. Luxembourg tax procedure imposes strict formal requirements, including short and mandatory time limits for filing a claim (réclamation) with the Director of the ACD, a mandatory pre-litigation phase preceding any judicial recourse, and particular rules as to the form, content and notification of pleadings. Failure to observe these requirements, even where the taxpayer’s substantive position is well-founded, frequently results in the claim being declared inadmissible, without the tax question itself ever being examined. This creates a genuine risk that taxpayers, or advisers unfamiliar with the technicalities of tax contentious procedure, focus on substantive arguments while overlooking procedural formalities that condition the very admissibility of their challenge.

2. The practical burden of proof

A second, closely related difficulty concerns the burden of proof. Under § 171 AO, taxpayers are required to substantiate the accuracy of their return and to prove assertions to the extent reasonably expected in the circumstances. In practice, discharging this burden can prove considerably more difficult, for a number of reasons.

• Historical documentation is incomplete or unavailable. Tax audits frequently concern transactions or facts that occurred several years prior, and taxpayers do not always retain supporting documentation for the full duration of the applicable limitation and audit periods.
• Factual allegations are inherently difficult to document after the fact. Certain matters, such as the actual place of effective management, or the reality and pricing of intra-group services, do not lend themselves easily to documentary proof, particularly where relevant evidence is held by third parties or located abroad.

Transfer pricing and restructuring transactions present a particularly acute illustration of this difficulty. Transfer pricing documentation, and evidence of the commercial and economic rationale underlying a restructuring, are frequently prepared, or finalised, only once a dispute has arisen or an audit has commenced, rather than at the time the relevant transaction took place. Documentation prepared retrospectively is inherently more vulnerable to challenge, as the tax authorities may regard it as reconstructed after the fact to justify a pre-determined tax outcome, rather than as a genuine, contemporaneous reflection of the taxpayer’s decision-making process.

Taxpayers are increasingly expected to demonstrate, through supporting contemporaneous evidence such as board minutes, feasibility studies and internal correspondence, that a given transaction or restructuring was driven by genuine business considerations rather than being principally tax-motivated, particularly in light of the general anti-abuse rule and the principal purpose test now embedded in Luxembourg’s tax framework. Where such documentation is absent or insufficient, taxpayers face a materially increased risk that the tax authorities will disregard the form of the transaction, re-characterise it, or deny the tax consequences claimed on the basis that the arrangement lacks genuine substance or economic justification.

3. Combined effect

These two difficulties compound one another. A taxpayer who has weak or incomplete documentation, whether general supporting records or, more specifically, contemporaneous transfer pricing and restructuring documentation, is already disadvantaged in discharging the burden of proof on the merits. If that same taxpayer also fails to observe the strict procedural framework applicable to any subsequent dispute, its position may be doubly compromised. This underscores the practical reality that successfully defending a position before the Luxembourg tax authorities and courts requires not only a sound substantive tax analysis, but also rigorous procedural discipline and a proactive, contemporaneous approach to the documentation of significant transactions, well before any dispute arises.

That combination makes procedural strategy unusually important: a taxpayer needs to consider not only whether an assessment is wrong, but also what parts remain open, what evidence exists, what the tax authorities can revisit, and whether challenging one point creates exposure elsewhere.