Tax return: the list of documents to gather
Skimming?
We've prepared the shortcut for you.
This is for you if
- You are a resident preparing your 2025 tax return or annual adjustment: the entire list may apply to you.
- You are a cross-border worker: for Form 163 NR, mainly your remuneration certificates; with tax assimilation (Form 100), add proof of household income from your country of residence.
Not for you if you are looking for what is deductible and up to how much: check out our deductions checklist.
Key figures
| Item | Figure | Year |
|---|---|---|
| Filing Form 100 and Form 163 | December 31, 2026 | 2025 Income |
| Automatic flat allowance for special expenses (full-year taxpayer) | €480 | 2025 Income |
| Donations: minimum annual total to deduct | €120 (cap: 20% of total net income) | 2025 Income |
| Childcare, declared domestic help: flat allowance (amount deducted from taxable income) | €5,400 per year, max €450 per month | 2025 Income |
Amounts: ACD (special expenses (ouvre dans un nouvel onglet), donations (ouvre dans un nouvel onglet), childcare (ouvre dans un nouvel onglet)). Deadlines: Guichet.lu (ouvre dans un nouvel onglet).
The checklist, situation by situation
For everyone
| Document | Where to find it |
|---|---|
| Annual remuneration certificate, one per employer | Your employer, who can provide you with a copy |
| Pension or annuity certificate | Your pension fund or paying organization |
| Foreign salary certificates, if you worked elsewhere during the year | Your former employer |
| IBAN in your name | Your bank |
| Tax withholding slip (fiche de retenue d'impôt - the ACD document showing your tax class to your employer) | Received by mail, or on MyGuichet.lu if you activated electronic delivery (eDelivery) |
Couples
| Document | Where to find it |
|---|---|
| Proof of partnership, if requesting joint taxation for the first time | Luxembourg civil partnership: certificate of registration in the civil register (répertoire civil), issued by the Parquet Général. French PACS or Belgian legal cohabitation: document from the registering authority, proving it existed all year |
| Married or civil partner cross-border worker requesting joint taxation (assimilation, Form 100): household income abroad | Spouse's employer, tax authority in your country |
Housing and savings
| Document | Where to find it |
|---|---|
| Annual interest certificate for your mortgage (mandatory for a loan taken out during the year) | Your bank |
| Invoices for loan origination fees (application fees, notary deed for loan, mortgage registration) | Bank, notary |
| Annual home savings statement (épargne-logement) | Your home savings bank |
| Pension savings premium certificate (111bis) | Your insurer or bank |
| Certificates for insurance premiums and personal loan interest | Your insurer, your bank |
Children
| Document | Where to find it |
|---|---|
| Invoices for crèche, day center, or certified childcare (child under 14 on January 1st), with proof of payment | Childcare provider, your bank |
| Salary slips for declared domestic help, or provider invoices | You, or the service company |
| Proof of costs paid for a child outside your household (bank transfers, court order) | Your bank, court |
Donations (€120 or more over the year)
| Document | Where to find it |
|---|---|
| Receipt or proof of each donation | The organization; check that it is listed on the ACD's list of approved organizations |
Cross-border workers
| Document | Where to find it |
|---|---|
| Luxembourg (and foreign) remuneration certificates | Your employers |
| Assimilation: proof of household income in your country of residence | Spouse's employer, your bank, tax assessment |
| Assimilation: record of days worked outside Luxembourg | Your employer, work-from-home schedule |
Beyond 50 days outside Luxembourg, these days count as foreign income for the 90% threshold (Guichet.lu (ouvre dans un nouvel onglet)): read our guide on tax assimilation. For commuting distance: travel expenses.
Submitting your documents
Create your MyGuichet.lu account
You must be at least 16 years old, have a 13-digit national identification number, and your own authentication method: LuxTrust (Mobile, SmartCard, or Scan, not a "light" certificate), eID, or eIDAS from your country (including France, Belgium, Germany). Registration must be done from a computer (Guichet.lu (ouvre dans un nouvel onglet)). Note: an eIDAS method does not allow you to electronically sign a procedure like a tax return (Guichet.lu (ouvre dans un nouvel onglet)).
Attach your supporting documents
In the MyGuichet.lu tax return wizard, PDF upload, or Form 163 questionnaire, attach your supporting documents. Not sure which form you need? Read our guide on tax return or annual adjustment.
Or send everything by mail
Form 100 to your tax office (bureau d'imposition), Form 163 to the RTS office (bureau RTS - tax withholding on salaries). The office mailbox works too; email does not (ACD (ouvre dans un nouvel onglet)). To find yours: which tax office.
Keep the originals
The ACD can request additional documents at any time, and a return filed via the assistant can be audited for 5 years (Guichet.lu (ouvre dans un nouvel onglet)).
A practical example
Let's take Camille, a resident, single with her 2-year-old daughter, one employer. In 2025: a mortgage for her apartment, €150 donated to an approved charity, €600 per month out-of-pocket for an approved crèche, €380 in deductible insurance premiums.
- Loan signed in 2025: bank interest certificate, mandatory. This loan is also why she files via Form 100 rather than an annual adjustment.
- Donation: €150, above the €120 minimum: she keeps the receipt.
- Crèche: €600 × 12 = €7,200 paid, but the deduction is capped at €450 per month, i.e., €5,400 (ACD). She attaches the invoices; the remaining €1,800 isn't covered by the flat-rate allowance (she can waive it and claim her actual costs as extraordinary expenses, a calculation worth comparing).
- Insurance: €380, under the €480 flat allowance granted automatically: submitting the certificate brings no additional tax benefit.
Common mistakes
- Providing only one remuneration certificate when you had two employers.
- Chasing insurance certificates for €200 in premiums: the €480 flat allowance already covers them.
- Counting on a single €80 donation: below the €120 total annual minimum, nothing is deductible.
- Cross-border worker: logging in with eIDAS and discovering at the moment of signing that it is not sufficient.
Frequently asked questions
Ask your employer for it: they issue it and can provide you with a copy (ACD). For a pension, contact your pension fund.
Certain documents must automatically be attached to your return: remuneration or pension certificate, interest certificate for a loan taken out during the year, proof of partnership for the first time (Guichet.lu, ACD). For others, keep them safe: the ACD may request them.
Guichet.lu asks for "proof of income" earned in your country of residence, without specifying a required document format. Keep whatever proves the amount: your spouse's annual salary statement, tax assessment if you already have it.