Tax return or annual adjustment: which one applies to you?
Skimming?
We've prepared the shortcut for you.
This is for you if
- You are a resident employee or pensioner and do not know whether you need to declare your 2025 income.
- You are a cross-border worker (France, Belgium, Germany): same thresholds, provided you worked in Luxembourg for 9 consecutive months.
- You have two jobs at the same time, rental income on top of your salary, or a partner to be taxed with.
Not for you if you only have income without tax withholding (self-employed): above €12,438 in taxable income, filing a tax return is mandatory (amount published by the ACD, page last updated on 28 October 2024).
Tax return or annual adjustment: the decision flowchart
Your taxable income is your gross salary after social security contributions, professional expenses, and special expenses (deductible private expenses: insurance, savings schemes...), so significantly less than your gross salary. The tax card is the ACD document indicating your tax class to your employer. The additional tax card is the one for your 2nd employer or 2nd pension when you receive two incomes at the same time: tax is withheld at a fixed rate, 33% in class 1, 21% in class 1a, and 15% in class 2 (ACD).
If you are a resident
| If… | Then… |
|---|---|
| your taxable income exceeds €100,000 | Form 100 mandatory |
| you have an additional tax card and over €36,000 in taxable income (classes 1 and 2), €30,000 in class 1a | Form 100 mandatory |
| you earn more than €600 net without tax withholding in addition to your salary (rent, side activity) | Form 100 mandatory |
| you receive more than €1,500 net in investment income (interest, dividends...) subject to Luxembourg withholding tax | Form 100 mandatory |
| in a civil partnership and wanting joint taxation; married and wanting individual taxation | Form 100 mandatory, joint request |
| you want to deduct mortgage interest on your home | Form 100, the annual adjustment does not take them into account (Guichet.lu (ouvre dans un nouvel onglet)) |
| none of these cases, but you probably overpaid | Form 163 R (or Form 100 on request) |
If you are a cross-border worker (non-resident)
| If… | Then… |
|---|---|
| 9 consecutive months in Luxembourg and over €100,000 in taxable income, or over €36,000 (€30,000 in class 1a) with an additional tax card | Form 100 mandatory |
| you want to be treated as a resident (tax assimilation) | Form 100, page 4, box 402 (ACD (ouvre dans un nouvel onglet)) |
| joint request for collective taxation (partners) or individual taxation (married couples) | Form 100 mandatory |
| none of these cases, but you probably overpaid | Form 163 NR |
Thresholds valid for your 2025 income (ACD, taxation by assessment (ouvre dans un nouvel onglet)). And if the ACD requests a tax return from you, you must file one.
Resident: three concrete profiles
2025 taxable income, 2025 ACD tax scale for tax class 1 (including the 7% contribution to the employment fund, excluding tax credits). These are general estimates.
Chloé, two part-time jobs at the same time. Employer A (main tax card): €1,700 taxable per month. Employer B (additional tax card): €1,200. Over the year, (1,700 + 1,200) × 12 = €34,800, under €36,000: no filing obligation. But employer B withheld 33% of €14,400, or around €4,750. With employer A's tax withholding (around €700), Chloé paid nearly €5,450 upfront, for an actual tax liability of around €3,170 on €34,800. An annual adjustment stands a good chance of refunding her the difference. If employer B paid her €1,500 per month, her total would be €38,400: mandatory tax return.
Paul, a salary and a rented studio apartment. €52,000 in taxable salary. Once expenses are deducted from rental income, his studio leaves him with €2,400 net over the year. That is more than €600 without tax withholding: mandatory tax return, even if far from €100,000.
Yasmine, executive. €104,000 in taxable income, single employer. Above €100,000: mandatory tax return, even if her monthly tax withholding was accurate all year long.
For cases without a mandatory filing obligation, our annual adjustment guide shows what you can get back.
Cross-border worker: Form 163 NR or tax return with assimilation
Luxembourg only taxes your Luxembourg-source income. Without a filing obligation, there are two paths.
The 163 NR adjustment allows few deductions: travel expenses not listed on your tax card, other professional expenses (frais d'obtention), social security contributions, and allowance (amount deducted from your taxable income) for children outside your household (Guichet.lu (ouvre dans un nouvel onglet)). If married, you remain in class 1: since 2018, obtaining class 2 for a married cross-border worker requires tax assimilation, hence Form 100 (ACD (ouvre dans un nouvel onglet)).
A tax return with assimilation treats you as a resident: special expenses, interest, extraordinary charges, single-parent tax credit (CIM). In return, you declare all household income, which is used to determine your tax rate. Condition: at least 90% of your worldwide taxable income is earned in Luxembourg (tested spouse by spouse), or less than €13,000 in net income not taxable in Luxembourg; for Belgian residents, also more than 50% of the household's professional income (ACD). Full details in our article on tax assimilation.
Example. Marc lives in Arlon and is single. In 2025, he earned €58,000 in taxable income from a single employer in Luxembourg, with no remote work. No filing obligation. He paid €3,000 into a private pension scheme, under the 2025 cap of €3,200 (Guichet.lu (ouvre dans un nouvel onglet)). A cross-border worker can only deduct this type of expense via Form 100 with assimilation (ACD); with 100% Luxembourg income, Marc is eligible. The assimilation simulator calculates the tax savings; without assimilation, the cross-border simulator estimates your tax overpayment.
Couples: when living together requires filing a tax return
Married residents: class 2 by default, a single annual adjustment for the household (Guichet.lu (ouvre dans un nouvel onglet)). Individual taxation requires a joint Form 100, signed by both spouses, submitted no later than December 31 of the following year (ACD (ouvre dans un nouvel onglet)).
Partners (PACS, Belgian legal cohabitation, Luxembourg civil partnership): class 1 each on their tax card. Class 2 is only accessible via a Form 100 signed by both partners, provided the partnership and shared address covered the entire calendar year, from January 1 to December 31 (ACD (ouvre dans un nouvel onglet)). The first time, attach the civil partnership declaration.
Example. Julie (€60,000 taxable) and Sam (€20,000), in a civil partnership since 2023, live together: for 2025, they can request collective taxation until December 31, 2026. With such an income difference, class 2 often reduces the tax bill: our article on collective or individual taxation helps you compare. If they had registered their civil partnership in June 2025, they would not qualify for 2025.
If your situation changes
- Marriage: between residents, collective taxation by default; individual taxation requires a joint Form 100.
- PACS / Civil partnership: no class 2 in the year of registration if it did not exist on January 1.
- Separation: update your tax card (form 164 R, or 164 NR if you are a cross-border worker, Guichet.lu (ouvre dans un nouvel onglet)).
- Second job: with an additional tax card, keep an eye on the €36,000 threshold (€30,000 in class 1a).
- Arriving in or leaving Luxembourg: the online assistant on MyGuichet.lu is unavailable to you that year, but you can file Form 100 through other channels (PDF via MyGuichet.lu, paper) (Guichet.lu (ouvre dans un nouvel onglet)), and your foreign income may affect your tax rate.
- Home purchase: to deduct mortgage interest, use Form 100.
Submitting your tax return or annual adjustment
Gather your documents
Salary tax certificate (certificat de rémunération: the annual salary summary provided by your employer), one per employer, supporting documents, international bank account number (IBAN): see our guide on documents to gather.
Choose your submission method
MyGuichet.lu (online assistant or PDF upload for Form 100, questionnaire for Form 163), postal mail, or the tax office drop box. Do not send by email: the ACD does not accept it.
Send to the correct tax office
Form 100: your competent tax office. If you are a cross-border worker, it is Luxembourg Y tax office (if residing in France), Z (Germany), X (Belgium and other countries). Form 163 R: the RTS tax office (withholding tax on salaries and pensions) responsible for your municipality. Form 163 NR: the RTS Non-residents tax office (ACD (ouvre dans un nouvel onglet)).
Respect the deadline
2025 income: from April 7 to December 31, 2026. In case of late filing, you risk a tax surcharge, late payment interest, or a penalty fine (Guichet.lu (ouvre dans un nouvel onglet)). Omission or error in your tax assessment notice? Write to your tax office quickly: a correction request only succeeds if the corrected notice is notified within 3 months of the first notice. To contest the notice, the objection goes to the Director of the ACD, also within 3 months of notification (caution: the notification date may be legally presumed: do not count from the day you open the letter).
Common mistakes
- Comparing your gross income to the thresholds: they apply to your taxable income.
- Submitting an annual adjustment when you have an additional tax card and over €36,000 in taxable income.
- Slipping mortgage interest into a Form 163: it requires Form 100.
- Thinking that a civil partnership automatically gives you class 2 on your tax card: it changes nothing during the year.
- Cross-border workers: expecting a 163 NR adjustment to grant deductions reserved for tax assimilation.
Frequently asked questions
Form 100 declares all your income and deductions, establishing your final tax liability. Form 163 only recalculates the tax on your salary or pension to refund an overpayment.
Yes, Guichet.lu allows filing on request. Useful if you have deductions that the annual adjustment does not accept.
No. The annual adjustment is reserved for those who do not file a tax return (ACD): if you submit Form 100, it handles your tax adjustment.
Yes, if your tax withholding was too low, you pay the difference. The annual adjustment, in principle, does not result in additional tax owed.