§ 100a AO assessments and claims: Outcome of the latest tribunal decision

The Luxembourg Administrative Tribunal has recently clarified the extent of the Director of Luxembourg tax authorities’ powers when ruling on claims against tax assessments issued under § 100a of the Abgabenordnung (AO). These are assessments based solely on the tax return submitted by the taxpayer, without a prior substantive audit, and they have become standard since electronic tax return filing was made mandatory.

In its decision of 19 November 2025 (48571), the Administrative Tribunal held that a taxpayer contesting such an assessment is entitled to a full examination by the Director under §§ 228 and 243 AO. The Director may not limit the review to simply verifying whether the assessment matches the original tax return. Furthermore, if a taxpayer submits amended annual accounts and an amended tax return after the Director’s decision, but before bringing the case before the tribunal, the tribunal may take these new elements into consideration and remit the case to the Director for a fresh examination.

This decision is important for taxpayers who discover errors in their tax returns after an assessment has been issued under § 100a AO. It confirms that the claim procedure is still fully available and that the Director is required to examine new facts that may influence the tax base, provided that the taxpayer files the claim within three months and provides complete information.

Factual background

On 25 April 2022, the taxpayer—a Luxembourg limited liability company—filed its corporate income tax, municipal business tax, and net wealth tax returns for the year 2020.

On 4 May 2022, the tax office issued the 2020 corporate income tax assessment, the 2020 municipal business tax assessment, and the net wealth tax assessment as of 1 January 2021, all on the basis of § 100a AO, in other words, “on the basis of the tax return,” and subject to a possible subsequent audit.

On 4 August 2022, the taxpayer submitted a claim with the Director of the Direct Tax Administration against the 2020 corporate income tax and municipal business tax assessments. The taxpayer claimed that its former adviser had made a material error in the 2020 annual accounts by including an unrealized foreign exchange gain as taxable profit. The taxpayer informed the Director that amended annual accounts would be submitted shortly.

On 22 November 2022, the Director rejected the claim as unfounded. While acknowledging that a claim is formally admissible against a § 100a assessment under § 228 AO, the Director held that, given the specific nature of § 100a AO, review is limited to verifying whether the assessment corresponds to the data reported in the tax return. In the Director’s view, a substantive assessment would conflict with the tax office’s right to conduct a subsequent audit and would undermine the principle under which a § 100a assessment becomes final after the five-year limitation period.

The Director also noted that, at the time of its decision, no amended annual accounts and no amended tax returns had been submitted.

On 22 February 2023, the taxpayer appealed the Director’s decision before the Administrative Tribunal. In the meantime, the taxpayer had filed amended annual accounts for 2020 on 17 February 2023, removing the disputed unrealized FX gain, and submitted an amended corporate income tax and municipal business tax return for 2020 on 20 February 2023.

The full extent of the Director’s powers in assessing a § 100a assessment

The core of the dispute concerned the extent of the Director’s powers when deciding on a claim against an assessment issued under § 100a AO.

The tribunal began by summarizing the content and purpose of § 100a AO. Under this provision, the tax office is authorized to issue an assessment solely on the basis of the taxpayer’s tax return, without a prior detailed examination, while retaining the right to conduct a subsequent audit. The legislative history confirms that this simplified approach is discretionary and that the tax office is free, but not required, to conduct a subsequent substantive audit and, where appropriate, to issue a diverging tax assessment.

The tribunal then noted that the legislator classified a § 100a assessment as a “formal assessment” within the meaning of § 211 AO. On this basis, established case law of the Administrative Court has already held that such assessments fall within the scope of § 228 AO and may be challenged by way of a claim before the Director in the same way as any other formal assessment.

The Director must consider all new arguments that might affect the tax base. The tribunal underlined that these powers are not restricted by § 100a AO, or any other provision, when the contested assessment is based on the tax return. In earlier decisions, the Administrative Court had also confirmed that the avenues of appeal against § 100a assessments are not narrower than for ordinary assessments and that, in the context of a claim, the taxpayer may amend the content of its tax returns to reduce the tax base.

Against this background, the tribunal rejected the Director’s position that his powers were limited to verifying whether the assessment matched the first tax return. A claim against a § 100a assessment must be subject to the same full review as a claim against any other formal assessment.

Consequences for the specific case and the taxpayer’s cooperation duties

Although the tribunal found the Director’s legal reasoning too restrictive, it acknowledged that it had already made a brief substantive assessment. It observed that the taxpayer’s claim merely stated the existence of a material error in the accounts (recognition of an unrealized FX gain) and announced the future filing of amended accounts and tax returns, but without providing supporting documentation or amended amounts.

The tribunal reiterated that the Director’s obligation to investigate a case is conditioned on the taxpayer’s cooperation. The taxpayer must submit sufficiently complete and quantified information with evidence to support it, to enable the Director to fully exercise his powers, including the possibility of increasing the tax base where appropriate. A taxpayer cannot invoke §§ 243 and 244 AO to force the Director to re-calculate the tax base on the basis of incomplete or merely declaratory statements.

In the case at hand, the tribunal noted that, at the date of the Director’s decision, the taxpayer had not yet submitted amended annual accounts or tax returns, nor any detailed quantification or supporting documentation. Under these circumstances, the Director was objectively unable to fully exercise his powers of taxation, and the dismissal of the claim as unfounded was, at that point, not manifestly incorrect.

However, the procedural circumstances changed after the Director’s decision. The taxpayer submitted amended annual accounts for 2020 on 17 February 2023 and an amended tax return on 20 February 2023, both before appealing to the tribunal on 22 February 2023.

In this context, the tribunal noted that:

• The taxpayer had already, in their original claim, made a clear indication of the nature of the alleged error;
• The taxpayer had subsequently submitted amended accounts and an amended tax return, which were now available in the court file; and that
• These new documents could have a significant impact on the tax base and thus required a thorough review.

In this case, the tribunal also reaffirmed that it is not intended to become a “tax assessor” and that it is not up to the court to substitute the tax administration by carrying out a full recalculation of the tax. The tribunal’s role is to interpret and apply the law, not to perform the administration’s primary assessment function.

In light of these considerations, the tribunal annulled the Director’s decision on appeal and referred the case back for further review. The Director is now required to review the amended annual accounts and the amended tax return, to assess the regularity of the tax treatment of the disputed unrealized FX gain, and to issue a new decision on the claim in view of the full information now available.

Key take-away

The decision in case 48571 confirms that claims against tax assessments issued under § 100a AO are entitled to the same full review by the Director as claims against ordinary assessments. The Director is not authorized to limit his review to a mere verification that the assessment matches the original return; he is required to review all facts and new elements submitted by the taxpayer and may then adjust the tax base accordingly.

The decision also emphasizes the taxpayer’s duty to cooperate. To enable the Director to exercise his full taxation power, including the possibility of reformation in pejus, the taxpayer must provide full and quantified information, rather than merely announcing future corrections.

Finally, the case highlights the relevance of the appeal in reformation before the Administrative Tribunal. The tribunal may take into account documents submitted after the Director’s decision and, if necessary, cancel the decision and refer the case back to the Director for a new decision based on a fully documented factual situation.