Luxembourg Tax Return: Form 100, Deadlines and Deductions

You must file a Luxembourg tax return (Form 100) if you fall within the assessment rules or the Luxembourg Inland Revenue (ACD) asks you to do so. Otherwise, an employee or pensioner who expects excess wage tax to be repaid will often be better served by an annual adjustment, Form 163. For the 2025 tax year, the filing window opened on 7 April 2026 and both routes are generally due by 31 December 2026.

Start by identifying the procedure, rather than assuming that filing a full return always produces a refund. A full assessment can establish tax still due; an annual adjustment is a request to correct tax already withheld and is designed for people who are not required to submit Form 100.

Do I need to file a tax return in Luxembourg?

Form 100 is the personal income-tax return used for taxation by assessment. The ACD publishes the detailed rules. For residents, common compulsory triggers include the following:

• only income not subject to withholding and taxable income above €12,438;
• taxable income above €100,000;
• more than €600 net income not subject to withholding in addition to income on which tax was withheld;
• certain salary paid by a foreign employer or other remuneration not subject to Luxembourg withholding;
• an additional fixed-rate tax card and taxable income above €36,000 in tax class 1 or 2, or €30,000 in class 1a;
• more than €1,500 of specified net capital income or directors' fees subject to withholding; or
• an explicit request from the ACD.

Marriage, partnership and individual-taxation elections can also require an assessment. Self-employed people and landlords should check their position particularly carefully: income without payroll withholding is a key reason Form 100 is required. The ACD's published assessment criteria are the authoritative starting point for a specific case.

A resident filing Form 100 reports worldwide income, including foreign income. A treaty may exempt that income in Luxembourg, but it can still influence the rate applied to Luxembourg-taxable income. The same full-disclosure principle applies when a non-resident opts for resident-equivalent taxation.

Should I use Form 100, annual adjustment Form 163 or a pre-filled return?

Use Form 100 where filing is compulsory, where you choose assessment, or where the claim needs a full return—for example, a resident seeking a tax adjustment for property-loan interest. It is available to residents and non-residents.

Use Form 163 R if you are a resident employee or pensioner, tax was withheld, and you are not subject to assessment. Use Form 163 NR for the corresponding non-resident route. Both can capture defined deductions and missing reliefs; they are not a substitute for a mandatory Form 100.

This procedure, available by request, cannot in principle result in any additional tax liability for the taxpayer.

That ACD guidance is why Form 163 can be the safer route where the aim is solely to correct over-withholding. Attach Luxembourg and, where relevant, foreign salary or pension certificates. The resident annual-adjustment rules and non-resident rules both set the deadline at 31 December of the following year.

The simple pre-filled return is different again. It is invitation-only, not a general filing method. The 2025 launch targeted taxpayers with only salary and pension income and no deductions beyond the minimum flat-rate; recipients must check and validate the proposal. Do not ignore a pre-filled proposal or assume it includes deductions you have not supplied.

How do I file a Luxembourg tax return online or on paper?

For tax year 2025, use the MyGuichet electronic assistant if its eligibility questions allow it. All income categories have been available through the assistant since tax year 2024. It guides the user through the return and supports attachments, but some situations—such as a change of country of residence during the tax year or certain cross-border joint-taxation elections—cannot use it.

• Gather annual salary or pension certificates, tax-card information and details of all income.
• Enter income, deductions and bank details carefully; include foreign income where the return requires it.
• Upload supporting documents and submit through MyGuichet, or complete, sign and upload the PDF Form 100.
• If filing on paper, send the signed return to the competent tax office.
• Keep the submission confirmation and your evidence. Electronically filed returns can be checked by the ACD for up to five years.

Whether online or by post, Form 100 is due by 31 December in the year after the tax year. Missing a deadline in an assessment case can lead to additional tax, late-payment interest or a penalty.

Which Luxembourg tax deductions can I claim, and what changes for non-residents?

Claim only costs that meet the conditions and that you can prove. Common entries include mandatory social-security contributions, qualifying work expenses, travel allowance, qualifying insurance premiums, consumer-loan interest, donations, private-pension and home-savings contributions, and eligible extraordinary expenses. Some may already have been reflected in withholding.

• Employees receive a €540 annual flat rate for other professional expenses; pensioners receive €300. Claim actual work costs only where they exceed the applicable flat rate and retain the breakdown and receipts.
• The travel allowance is €99 per distance unit after the first four units, capped at 26 units or €2,574 a year.
• For the relevant insurance-premium and consumer-credit-interest category, the 2025 ceiling is €672, increased by the same amount for a spouse and each child in the household.

For a non-resident, the first question is whether to request treatment equivalent to a resident on Form 100. That can open the same deductions, allowances and credits, but requires declaring worldwide income. The usual gateway is at least 90% of income taxable in Luxembourg, or net income not taxable in Luxembourg below €13,000; Belgian residents may alternatively use the 50% household professional-income test. The first 50 foreign workdays may be neutralised only for the 90% calculation. Without assimilation, deductible expenses are limited; Form 163 NR remains available in defined cases, including specific rules for workers whose Luxembourg gross pay exceeds 75% of annual earned income.

Keep annual income certificates, invoices and payment proofs, insurance and pension certificates, loan-interest statements, donation receipts, travel and employment records, and documents supporting family or residence status. A return can reduce over-withholding, but its result depends on the whole tax position—not the presence of a deduction alone.