Form 100 or Annual Adjustment? Choosing the Right Luxembourg Tax Filing
Updated: Oct 18
Luxembourg employees often use the expressions "tax return" and "annual adjustment" as if they mean the same thing. They do not. Form 100 and Form 163 are different procedures with different eligibility rules, scope and possible outcomes.
Choosing the correct route is the first step in preparing a filing.
Form 100: a full income tax assessment
Form 100 is the Luxembourg personal income tax return used for taxation by assessment. It covers the taxpayer's relevant income categories, deductions, allowances, tax credits and family circumstances.
Form 100 must be used when assessment is mandatory. It can also be filed voluntarily in permitted cases, including to request tax options or deductions that are unavailable through the annual adjustment procedure.
Because it establishes the final income tax for the year, the result may be:
• a refund;
• no material balance; or
• additional tax to pay.
A taxpayer should therefore not assume that filing Form 100 is automatically a refund request.
Form 163: adjustment of wage or pension withholding
Form 163 is designed for employees and pensioners who had Luxembourg tax withheld but are not subject to assessment. It compares the tax calculated for the year with the amount withheld by the employer or pension fund.
If too much was withheld, the ACD refunds the difference. According to the administrative guidance, the procedure cannot in principle create an additional tax liability. It is therefore mainly useful where there is a reasonable expectation of a refund.
Residents use Form 163 R; non-residents use Form 163 NR.
What can a resident claim through Form 163?
A qualifying resident employee or pensioner may use the annual adjustment to claim items such as:
• eligible work-related expenses not fully reflected in payroll;
• special expenses, including qualifying insurance and pension contributions;
• extraordinary expenses;
• certain child-related relief or the single-parent tax credit;
• correction for periods of work and inactivity or excess withholding.
The necessary annual salary and pension certificates must be attached.
When Form 100 is required instead
Form 100 is generally the relevant route where:
• the taxpayer meets a mandatory assessment condition;
• mortgage interest connected with an owner-occupied property is claimed;
• rental income or a rental loss must be reported;
• foreign income must be included as part of a full assessment;
• a non-resident requests treatment equivalent to that of a resident;
• spouses or partners request an assessment option that requires a return;
• income from self-employment, director fees, investments or other sources falls within the assessment rules.
The owner-occupied mortgage rule is particularly important: Guichet.lu expressly directs resident taxpayers claiming this interest to Form 100 rather than Form 163.
Non-residents: a narrower annual adjustment
A non-resident who is not subject to assessment may request an annual adjustment, but the deductions available through Form 163 NR are limited. Under specified circumstances, foreign income must also be stated to determine the rate.
A non-resident seeking the broader deductions available to residents may need to file Form 100 and request fiscal assimilation. Eligibility commonly depends on the proportion of worldwide income taxable in Luxembourg or the amount of foreign income.
Filing method and deadline
For tax year 2025, both procedures can be submitted using the relevant MyGuichet.lu service. PDF or paper options are also available where applicable.
The general deadline for both the 2025 Form 100 and a 2025 annual adjustment is 31 December 2026. Any specific earlier deadline communicated by the ACD should be respected.
Quick comparison
Choose Form 100 when: a full assessment is mandatory, a specific tax option is requested, mortgage or rental-property items are involved, or worldwide income must be considered under an assessment.
Consider Form 163 when: you are an employee or pensioner outside the assessment rules, only a limited withholding correction is needed and the objective is to recover excess tax deducted at source.
Review the position before choosing when: there are multiple employers, foreign workdays, foreign income, a spouse earning abroad, a change of residence or an uncertain filing obligation.
How Samstein Advisors can help
Samstein Advisors can determine the appropriate filing route, estimate the likely outcome and prepare the relevant submission with the supporting documents required for your circumstances.
This article is based on guidance available on 13 August 2026. Filing eligibility depends on the relevant tax year and individual facts. This is general information, not personalised tax advice.
Official sources
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