10 Common Luxembourg Tax Return Mistakes — and How to Avoid Them
Updated: Oct 18
Most tax-return mistakes are not caused by complex calculations. They arise because information is incomplete, a filing route is misunderstood or an amount is entered without considering the rule behind it.
Here are ten recurring errors to check before submitting a Luxembourg personal income tax return.
1. Assuming that pre-filled information is complete
Electronic and simplified returns can include salary and pension information, but they do not remove the taxpayer's responsibility to check the filing. Foreign income, family changes, property, deductions and workdays outside Luxembourg may require additional information.
Avoid it: Compare the pre-filled data with annual certificates and your complete personal situation for the year.
2. Using Form 163 when Form 100 is required
The annual adjustment is not a shorter substitute for every tax return. It is available to employees and pensioners outside the assessment rules and has a limited scope. Owner-occupied mortgage interest, rental matters and certain tax elections require Form 100.
Avoid it: Confirm the filing obligation and required procedure before entering figures.
3. Leaving out foreign income because it is taxed abroad
Luxembourg residents generally report worldwide income. Income that is exempt in Luxembourg under a treaty can still be used to determine the rate applied to Luxembourg-taxable income. Non-residents requesting treatment as residents also report worldwide income for this purpose.
Avoid it: Identify every country and every income source, then apply the treaty method rather than omitting the income.
4. Reporting only one spouse's income
Joint taxation is based on the relevant income of both spouses or partners. A spouse's foreign salary, pension, investment or rental income may affect the global rate even if it is not taxed directly in Luxembourg.
Avoid it: Collect income evidence for both taxpayers before calculating the return.
5. Claiming the payment instead of the deductible amount
Many deductions are capped or subject to a standard allowance. Examples include private pension contributions, insurance, home savings, work expenses and childcare. Other claims are reduced by reimbursements.
Avoid it: Determine the eligible category first, then apply the tax-year limit, household multiplier and reimbursement rules.
6. Treating mortgage principal as deductible interest
A mortgage payment usually contains principal and interest. The principal repayment is not deductible as interest. The deductible interest may also be reduced by subsidies and limited according to the property's stage and occupation period.
Avoid it: Use the bank's annual interest certificate and property timeline rather than the total monthly payments.
7. Mixing the tax year with the filing year
A return filed in 2026 for tax year 2025 uses the income, deductible payments and rules relating to 2025. A certificate or legal limit applicable from 2026 should not automatically be applied to the 2025 return.
Avoid it: Label every working paper with the tax year and verify effective dates. For example, the private-pension deduction ceiling is EUR 3,200 for 2025 and increases to EUR 4,500 only from 2026.
8. Ignoring telework and business-travel days
For cross-border workers, days worked outside Luxembourg can affect where salary is taxable. The 34-day treaty tolerance applicable to residents of France and Germany counts relevant workdays outside Luxembourg, not only days informally described as home office.
Avoid it: Maintain a contemporaneous calendar showing location, activity and supporting travel or employer records.
9. Submitting weak or inconsistent evidence
A deduction can be delayed or rejected when the certificate does not identify the taxpayer, period, amount or nature of payment. Problems also arise when an invoice conflicts with the amount entered or when only a payment screenshot is provided without the underlying document.
Avoid it: Reconcile each claimed amount to a clear annual certificate, invoice and proof of reimbursement where relevant.
10. Failing to review the assessment notice
Filing is not the end of the process. The ACD may adjust an amount, request information or issue an assessment that differs from the return. Missing the applicable response or objection deadline can make correction more difficult.
Avoid it: Compare the assessment notice line by line with the submitted return as soon as it arrives, and respond within the deadline shown or legally applicable.
Final quality-control questions
Before submission, ask:
• Have all income sources and countries been considered?
• Does the filing route match the taxpayer's legal position?
• Are 2025 amounts supported by 2025 certificates?
• Have reimbursements and subsidies been deducted?
• Are both spouses' or partners' details complete?
• Are all required signatures or mandates present?
• Has a copy of the submission and receipt been retained?
A short review using these questions can prevent a long exchange with the tax office later.
How Samstein Advisors can help
Samstein Advisors reviews the return as a complete file rather than as a collection of isolated numbers. This helps identify inconsistencies between income, family circumstances, deductions, foreign information and supporting evidence before submission.
This article is based on guidance available on 13 August 2026. It provides general information and does not constitute personalised tax advice.
Official sources
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