Form 100 Luxembourg: compulsory for some, useful for others.
Luxembourg withholds tax on the salary every month, based on the tax withholding card. With a single employer, the story often ends there: nothing to file in the Grand Duchy.
The return becomes compulsory in the cases the Administration des contributions directes (ACD) lists on its page on taxation by assessment, updated on 28 October 2024. For a non-resident employee taxed at source for at least nine months, form 100 is required as soon as taxable income exceeds €100,000, or as soon as there are several withholding cards (two employers, or a salary and a pension) and the total exceeds €36,000 in class 1 or 2, €30,000 in class 1a. It is also required above €1,500 of net investment income or directors' fees (tantièmes) subject to withholding, for Luxembourg income with no withholding at all, and when the administration asks for it. Two choices also make the filing compulsory: the request for assimilation to a resident, and the option taken by couples for joint or individual taxation.
Assimilation is what makes form 100 useful for a frontalier who is under no obligation. The guichet.lu page on the option for tax treatment equivalent to a resident's, updated on 23 September 2025, sets the central condition: at least 90% of worldwide income taxable in Luxembourg. Two side doors: under €13,000 of net income not taxable in Luxembourg, or, for a Belgian resident, 50% of the household's professional income taxable in the Grand Duchy. Days worked outside Luxembourg but exempt count as Luxembourg days, up to 50 of them.
In return, the same page promises the deductions, allowances and credits of a resident, mortgage interest and extraordinary expenses included. The request is a box ticked in form 100 itself.
For an employee with nothing but a salary to settle, the annual adjustment (décompte annuel), form 163 NR F, is the short route. The guichet.lu page on the annual adjustment for non-residents, updated on 7 April 2026, reserves it for employees outside taxation by assessment, subject to a condition on the length of activity (nine months) or on the share of the Luxembourg salary in professional income (75%), and as a rule it cannot end in additional tax.
| Situation of the employed frontalier | Form 100 |
|---|---|
| One employer, taxable income under €100,000, nothing else | No obligation |
| Taxable income above €100,000 | Compulsory |
| Two withholding cards, total above €36,000 (classes 1 and 2) or €30,000 (class 1a) | Compulsory |
| Luxembourg income with no withholding at source | Compulsory |
| Request for assimilation (90% of income in Luxembourg, or under €13,000 elsewhere, or 50% of the household for a Belgian resident) | Optional, then compulsory once requested |
| Couple opting for joint or individual taxation | Optional, then compulsory once requested |
| Employee under no obligation who wants to correct withholding that was too high | Annual adjustment (form 163 NR F) |
In Luxembourg, withholding at source is the rule and form 100 the exception: triggered by a threshold or requested for a deduction, never automatic.
Declare a Luxembourg salary in France: it comes back on form 2042.
On the French side, the annual return is due from every resident, whatever country pays. What remains is where the Luxembourg salary is entered, and what it sets off.
The mechanism comes from the tax treaty of 20 March 2018, completed by the amendment of 10 October 2019 and applicable since 1 January 2020. The DGFiP step-by-step guide on applying the Franco-Luxembourg treaty to 2024 income records the switch: the old effective rate method, tolerated for 2020 to 2023 income, ended with 2024 income. Since then, the Luxembourg salary gives rise to a tax credit equal to the French tax.
In practice, the salary enters French taxable income. Tax is calculated on all of the household's income, then a credit equal to the share of French tax that corresponds to that salary cancels it. The notes to form 2047 for 2025 income state that this credit is calculated whatever the tax actually paid in Luxembourg, and that "French tax means income tax plus social levies". The salary therefore produces no French tax of its own, but it weighs on the rate applied to the household's other income, a spouse's French salary for example.
The amount declared is the gross salary, without deducting the tax paid in Luxembourg; the flat 10% deduction then applies by itself. The step-by-step guide gives the example of a couple: Dominique, employed in Luxembourg, €40,000 of salary and €2,000 of Luxembourg tax; Camille, employed in France, €25,000.
| Item declared | Form | Box | Amount in the example |
|---|---|---|---|
| Luxembourg salary, detail | 2047, section 1 | Salaries and wages | €40,000 |
| Luxembourg salary, tax credit | 2047, section 6 | Income giving rise to a credit equal to the French tax | €40,000 |
| Luxembourg salary, carried over | 2042 | 1AF (declarant 1) or 1BF (declarant 2) | €40,000 |
| Total Luxembourg income with credit | 2042 | 8TK | €40,000 |
| Spouse's French salary | 2042 | 1BJ | €25,000 |
| Tax paid in Luxembourg | none | not deducted | €2,000 |
For anyone who works remotely from France, the treaty grants a flat 34 days a year outside Luxembourg without changing the country of taxation. Beyond that, the share of salary that corresponds to the days in France becomes taxable in France; the step-by-step guide indicates that it is pre-filled in box 1AG, without the credit. These box numbers are those of the 2026 campaign, and the notes to form 2047 for the current year are what counts.
In France, the Luxembourg salary is declared gross, neutralised by a credit, and never leaves the calculation of the rate.
Luxembourg tax return for a non-resident of Belgium or Germany: same logic, other forms.
A Belgian resident also declares all worldwide income, Luxembourg salary included. The SPF Finances says so on its page on income from abroad: even when a treaty takes the right to tax it away from Belgium, that income appears in the return, because it sets the rate on other income and, in some cases, the municipal tax. The method is called exemption with progression (exonération avec réserve de progressivité), described on the page on living and working in different EU member states.
The Belgian tolerance threshold is 34 days a year outside Luxembourg. The ACD note on cases covered by the Belgo-Luxembourg treaty, dated 2 March 2023, ties it to the amendment of 31 August 2021, which entered into force on 10 February 2023 and applies to remuneration from 1 January 2022. Before it, the threshold was 24 days.
Same figure for a German resident. The protocol of 6 July 2023 amending the German-Luxembourg treaty and the mutual agreement of 11 January 2024 published by the federal Ministry of Finance (Verständigungsvereinbarung zur Anwendung und Auslegung des DBA Luxemburg) set, since 1 January 2024, 34 working days per calendar year, with no pro rata for part-time work. In Germany, the exempt Luxembourg salary goes into the annual return under the Progressionsvorbehalt, the progression clause.
Belgium, France or Germany: the country of residence does not take the tax on the Luxembourg salary, but it uses that salary to set the rate on everything else.
Luxembourg tax return deadline and documents: what to keep, what to file when.
The paperwork is short. The ACD's frequently asked questions for non-residents gives the exact names.
- The certificate of remuneration (certificat de rémunération), or certificate of salary, tax withheld and tax credits, issued by the employer at the start of the year. It goes with the annual adjustment as it goes with form 100.
- The tax withholding card (fiche de retenue d'impôt), which sets the class and the month's rate. Changing it goes through form 164 NR, with supporting documents.
- The statements behind a deduction requested under assimilation: mortgage interest, insurance premiums, extraordinary expenses.
- For France, forms 2047 and 2042 as filed, with the certificate of remuneration behind the amount entered in box 1AF.
The dates barely move in Luxembourg and change every year in France. The guichet.lu page on the income tax return, updated on 7 April 2026, sets the limit at 31 December of the year that follows the tax year, and places the filing for 2025 income between 7 April 2026 and 31 December 2026. After that, the tax office can apply a tax surcharge, late interest or a penalty payment, and no extension is granted. Filing is done as a PDF via MyGuichet.lu or by post.
For France, the impots.gouv.fr page on how the 2026 income tax return is filed gives the deadlines by département and the date for paper.
| Deadline | Date | Source |
|---|---|---|
| Form 100, 2025 income | 7 April 2026 to 31 December 2026, no extension | guichet.lu, ACD |
| Annual adjustment (form 163 NR F), 2025 income | 31 December 2026 | guichet.lu |
| French return online, 2025 income | 21 May 2026 (départements 01 to 19 and non-residents), 28 May (20 to 54), 4 June (55 to 974 and 976) | impots.gouv.fr |
| French return on paper, 2025 income | 19 May 2026 | impots.gouv.fr |
| Belgian and German returns | The date is published each year by the SPF Finances and by the Finanzamt | fin.belgium.be, bundesfinanzministerium.de |
Luxembourg leaves until 31 December of the following year and does not extend; France closes in May or June and publishes its dates each spring.
Frequently asked questions
Does a frontalier file a tax return in Luxembourg?
Not by default. Withholding at source settles the tax of a non-resident employee with one employer and taxable income under €100,000. Form 100 becomes compulsory above that, with several withholding cards above the thresholds, for income with no withholding, or once an assimilation or a couple's option has been requested.
How does a frontalier fill in the Luxembourg tax return?
Form 100 is filed as a PDF via MyGuichet.lu or by post to the competent tax office, with the certificate of remuneration. Assimilation is requested in the form itself, and it is what opens the deductions. For an employee under no obligation who only wants to correct withholding that was too high, the annual adjustment (form 163 NR F) is enough.
How do I declare a Luxembourg salary in France?
The gross salary, without deducting the Luxembourg tax, goes in sections 1 and 6 of form 2047, then in boxes 1AF (1BF for the second declarant) and 8TK of form 2042, with a tax credit equal to the French tax. The share of remote days worked in France beyond the 34 days is declared separately, in box 1AG, without that credit.
When is the Luxembourg tax return deadline?
No later than 31 December of the year that follows the income year, which means 7 April 2026 to 31 December 2026 for 2025 income. The ACD grants no extension and goes by the date of arrival at the tax office, not the date of posting.
Key points
- An employed frontalier with a single employer and taxable income under €100,000 has nothing to file in Luxembourg; form 100 becomes compulsory above that, with several withholding cards, with income that carries no withholding, or once an option has been requested.
- Assimilation to a resident is requested in form 100 and rests on 90% of worldwide income taxable in Luxembourg, under €13,000 of net income elsewhere, or 50% of the household for a Belgian resident.
- In France, the Luxembourg salary is declared gross on form 2047, then in boxes 1AF and 8TK of form 2042, with a tax credit equal to the French tax; beyond 34 days of remote work, the French share goes in 1AG.
- Belgium and Germany exempt the Luxembourg salary with progression and apply the same 34-day threshold.
- Form 100 for 2025 income is filed between 7 April and 31 December 2026 with no extension; the French return closes between 19 May and 4 June 2026.