What happens after I submit my declaration via the MyGuichet assistant?

When you submit your tax return via the MyGuichet assistent, the ACD applies a simplified procedure known as ‘assessment based on the return’, as provided for in Section 100a of the General Tax Code (Abgabenordnung). You will also find this reference directly on your tax notice, usually worded as follows: “Imposition suivant déclaration (§100a AO) sous réserve d'un contrôle ultérieur / Besteurung nach Erklärung (§100a AO) unter Vorbehalt späterer Prüfung”.

In practical terms, here is what this means for you:

A quick, but not final, tax assessment

The tax assessment is usually issued within 15 days of the return being submitted. This speed comes at a cost: the calculation is based solely on the information entered, without any ACD officer reviewing the file at this stage. This initial statement is therefore issued immediately, but remains provisional — it is the provisional statement.

No review at the provisional statement stage

When the provisional statement is issued, no human verification beyond the technical consistency of the completed fields is carried out. No agent checks whether the deductions to which the taxpayer may be entitled have actually been claimed: travel expenses, mortgage interest, insurance, pension contributions, extraordinary expenses, acquisition costs, donations… A missed box or a misunderstood section has a direct impact on the tax calculation, and the provisional statement reflects exactly what has been entered, no more and no less.

The verification takes place when the assessment becomes final

The situation changes when the ACD issues the final assessment. At that point, the file is examined by a tax office officer, who reviews the tax return and supporting documents before setting the final tax liability. This transition from provisional to final can occur at any time during the five years following the issue of the provisional assessment, and the ACD is not required to justify when it decides to finalise a file: this may happen at random, following a report, or whilst processing another tax year.

Two possible outcomes when moving to the final assessment

The ACD’s review of the case can, in theory, go either way:

• The unfavourable scenario: if an error, an omission or incorrectly declared income is detected, the ACD issues a final assessment against the taxpayer, with additional tax payable.

• The favourable scenario: if, during the verification, a forgotten deduction or an error to the taxpayer’s disadvantage is identified, the ACD may issue a final assessment in the taxpayer’s favour, resulting in an additional refund. This scenario is, however, contingent on the file actually being re-examined in detail.

Whether the statement is provisional or final, it is always possible to lodge an appeal with the Director of the ACD within three months of receiving notification of the statement in question.

In practice:

• Appeal against the provisional statement: this is useful as soon as you realise that a deduction has been omitted, income has been incorrectly declared, or a change in family circumstances has not been taken into account (change in tax bracket, birth, etc.). If no appeal is lodged within the three-month period, the statement remains in force until it becomes final.

• Appeal against the final tax assessment: a new three-month period begins upon notification of the final assessment. This is your opportunity to challenge an additional tax liability deemed unfounded, or to report an omission that has still not been corrected by the ACD.

In both cases, you must still identify the error or omission yourself. And this is exactly where taxx.lu can help: our teams analyse the statement received (provisional or final), compare it with your actual situation, identify any errors or missing deductions, and prepare the claim to be submitted to the ACD within the 3-month deadline.

The classic unpleasant surprise

Here is the most common scenario:

A tax return is submitted via the MyGuichet assistent. A few days later, the provisional statement shows a refund of €1,200. The money arrives in the account and is spent accordingly.

Two years later, the tax office finalises the case. It finds that non-deductible expenses were declared in error, or that income was not correctly reported. A final statement is then issued: not only must the €1,200 refund be repaid, but an additional €600 in tax is also due.

The result: what looked like a welcome refund turns into a tax debt of nearly €2,000, to be settled within a short timeframe.

Updated on: 26/05/2026