French tax declaration: how and why it may be a good idea to make a ‘mention expresse’

Deadlines to file your 2025 tax (for 2024 income) are fast approaching in France, falling as early as next week depending on where you live. 

Declarations can be a complex process, particularly when users are calculating their own deductions and expenses they believe they are entitled to.

It leaves many stressed that they have perhaps made an error and in turn will have to pay financial penalties for doing so if they are found to have under-declared.

However, declarants are able to use the mention expresse to help avoid this scenario by being honest and upfront with the tax authorities about calculations they have made. 

Article 1727 of the French tax code establishes the principle that a declarant using a mention expresse will not have to pay any late interest fees on any recalculated tax amount (they do, however, have to pay any extra tax due, of course).

This applies both to cases where the user changes figures pre-filled by the tax office or where they declare – or deliberately do not declare – other income sums, or for example they claim certain deductions from taxable income.

In cases where the authorities need to recalculate your tax because you made a mistake and this results in more tax to pay, then interest at 0.2% per month (of the extra tax payable) is usually payable. 

However, this will usually be waived if the extra tax relates to something you drew attention to in a mention expresse. 

How do I use the mention expresse? 

The mention expresse is made at the end of your declaration right before signing or confirming the information, when you are asked by the authorities if you have anything to add. 

If declaring online, you can select a box to write a mention expresse for the authorities to see. 

If declaring on paper, you can use the Informations box on page two of the 2042 form, or write it on a separate sheet of paper, clearly writing ‘Mention expresse’ at the start.

If you have a general query about a deduction or change you plan to make, and have time to do so, it is best to ask about it via the secure messaging system (messagerie sécurisée) in your personal space on the tax site prior to submitting your return, or by calling the tax office helpline (0809 401 401) or making use of a service such as Allô-Impôt. 

Note that if you discover an error after submission, you can correct your declaration as many times as you wish before your deadline. 

If, however, you wish to make use of the mention expresse during your declaration, then explain the actions you have taken, and why in your note.

Add comments to the effect that “I do not believe that [this - detail what] income needs to be declared” or “I believe that I can deduct [this - detail what] sum because…”.

For example, if you have deducted €10,000 in expenses from your salary in box 1AJ, state this and explain why you believe this to be correct.

In the event you are wrong, and the tax office is satisfied that it was a genuine and understandable mistake made in good faith, then there will be no penalties.