Luxembourg form 100: who must file it, and how to fill it box by box
Form 100 is the Luxembourg tax return, for residents and cross-border workers alike. Generous deadline: December 31 of the following year. Even when not mandatory, filing often recovers withheld tax through deductions (insurance, pension savings, interest…).
Mandatory or voluntary?
Filing becomes mandatory above certain income thresholds or in specific situations (multiple incomes, income not subject to withholding…). But many employees who are NOT required to file benefit from doing so voluntarily: it’s the only way to claim deductions (outstanding-balance insurance, pension savings, loan interest, childcare…) and recover part of the withholding.
Cross-border workers: access to most deductions requires “resident assimilation” — at least 90% of household income from Luxembourg sources (or under €13,000 of other income) for a French resident, more than 50% for a Belgian resident.
The form in practice
Form 100 runs about twenty pages, but each is numbered and every field has a box number — your salary comes from the employer’s “salary, tax withholding and tax credits” certificate, withholding carries to boxes 1901/1902, insurance and home-savings have their dedicated pages.
File online on MyGuichet.lu (guided assistant) or on paper. Deadline: December 31 of the year after the income year — the 2025 return can thus be filed until December 31, 2026.
Every box, pre-filled for you
Our flows (resident, French cross-border, Belgian cross-border) turn your answers into a box-by-box recap of form 100 — page number, box number, exact amount — ready to copy into MyGuichet.
Frequently asked questions
Tax was already withheld at source: what’s the point of filing?
To recover the overpayment: withholding ignores your insurance, pension savings and loan interest. For many employees, voluntary filing results in a refund.
Cross-border worker: do I file in Luxembourg AND France?
Yes, both — form 100 on the Luxembourg side, and the 2042 + 2047 on the French side (the salary qualifies for a credit equal to French tax: no double taxation, but filing remains mandatory).