Tax assimilation: being taxed like a resident
Skimming?
We've prepared the shortcut for you.
It's for you if
- You live in France, Belgium, or Germany and you are an employee or retiree in Luxembourg.
- Your spouse earns little, nothing, or less than you outside Luxembourg.
- You pay expenses that a non-resident cannot deduct: private pension scheme, home-ownership savings scheme. Mortgage interest for a house in your home country also counts, but only to lower your tax rate.
- You are a single parent: without assimilation, the single-parent tax credit (CIM) remains inaccessible to a non-resident (ACD).
Not for you if you live in Luxembourg, or if your spouse earns significantly more abroad than you do in Luxembourg.
Conditions, country by country
| Your situation | Condition (only one is needed) | Year |
|---|---|---|
| You live in France or Germany | At least 90% of your global income is taxable in Luxembourg | 2025 and 2026 |
| You live in France or Germany | Net non-taxable income in Luxembourg under €13,000 | 2025 and 2026 (since 2018) |
| You live in Belgium | Both conditions above, or more than 50% of the household's professional income taxable in Luxembourg | 2025 and 2026 |
| Days worked outside Luxembourg | For the 90% test, up to 50 days of salary taxed elsewhere count as Luxembourgish | 2025 and 2026 |
The 90% and €13,000 tests are conducted spouse by spouse: if one meets the criteria, the couple can request assimilation (ACD Non-Residents FAQ (ouvre dans un nouvel onglet)). The decision scheme:
- Is at least 90% of your income taxable in Luxembourg? Eligible.
- Otherwise, is your net non-taxable income in Luxembourg under €13,000? Eligible.
- Otherwise, do you live in Belgium and does the household exceed 50%? Eligible.
- Otherwise, does your spouse pass test 1 or 2? The couple is eligible. Otherwise, you remain a non-resident, in Class 1 if you are married (ACD (ouvre dans un nouvel onglet)).
Cross-border worker in France: the 50-day rule
Take Sophie in Thionville: €55,000 in taxable income for 220 working days, including 60 teleworking days in France. Since she exceeds 34 days, these 60 days become taxable in France (see the 34-day rule). To simplify, we count €250 per day.
- 90% test: the first 50 days count as Luxembourgish (ACD). 10 days remain, i.e., €2,500 foreign: €52,500 / €55,000 = 95.5%, passed.
- €13,000 test: 60 days taxed in France, i.e., €15,000, failed. A single test is enough.
At 80 teleworking days, 30 days (€7,500) would count as foreign: 86.4%, and €20,000 non-taxable in Luxembourg. No more assimilation, except via the spouse. This 50-day rule is used only for the 90% test: it does not change the country that taxes these days (Guichet.lu).
Couples where the spouse works in France
Take Marc in Longwy, with €60,000 of taxable income in Luxembourg. Without assimilation, a married non-resident is taxed in Class 1 on their Luxembourg income only (ACD): €11,945, employment fund (7% contribution added to the tax) included. With assimilation, the ACD calculates a Class 2 tax rate on the couple's global income (the splitting mechanism (ouvre dans un nouvel onglet)), then applies this overall rate (calculated on all household income, applied to your Luxembourg income only) to Marc's €60,000. It all depends on Julie's salary in France:
| Julie's income in France | Overall rate | Marc's tax | Difference compared to Class 1 |
|---|---|---|---|
| €0 | 7.13% | €4,275 | €7,670 less |
| €30,000 | 13.49% | €8,096 | €3,849 less |
| €60,000 | 19.91% | €11,945 | None |
| €80,000 | 23.03% | €13,815 | €1,870 more |
trigger calculation, ACD 2025 tax scale, before deductions. The break-even point here occurs when Julie earns as much as Marc.
Cross-border worker in Belgium: the 50% household rule
Take Thomas in Arlon: €45,000 of taxable income in Luxembourg, €15,000 from an activity in Belgium, and a spouse with no income. 90% test: 75%, failed. €13,000 test: failed. But 75% of the household's professional income is taxable in Luxembourg, which is more than 50%: Article 24 § 4a of the Belgium-Luxembourg double tax treaty grants him assimilation. However, his €15,000 from Belgium will increase his tax rate.
Cross-border worker in Germany: eligible doesn't mean advantageous
Take Jonas, single in Trier: €48,000 taxable in Luxembourg and €9,000 in net rental income in Germany. A ratio of 84.2%, but under €13,000 non-taxable in Luxembourg: he is eligible. Without assimilation, he pays €7,116 (Class 1). With it, his tax rate is calculated on €57,000 (18.76%) and his tax rises to around €9,005 (trigger calculation, 2025 scale, before deductions). For him, the option is only worthwhile if his resident deductions offset the difference.
If your situation changes
- Marriage: without assimilation, each spouse remains in Class 1; with it, the couple's tax rate is calculated in Class 2. To adjust tax withholding this year: Model 166 or online process on Guichet.lu (instructions for Model 164 NR 2026 (ouvre dans un nouvel onglet)).
- Civil partnership (PACS): joint taxation only upon joint request (Model 100, box 404, with the assimilation request in box 402), provided the partnership and shared residence cover the entire year (details in joint or individual taxation).
- Separation: each person re-takes the test; the ACD details the tax classes for the year of divorce in its non-residents table.
- Second job in your home country: your foreign income increases, re-take the tests.
- Moving to Luxembourg during the year: box 401 of Model 100 allows you to request taxation as a resident for the entire year.
- Moving from France to Belgium: conditions and tax office change (see find your tax office).
How to request assimilation
Gather household income
Luxembourg remuneration certificate and foreign income for both spouses, expressed in net amounts recalculated according to Luxembourg rules.
Check box 402 on Model 100
In Model 100 for the year 2025 (ouvre dans un nouvel onglet), page 4, section "Assimilation du non résident au résident". Luxembourg income in the "revenus non exonérés" column, foreign income in "revenus exonérés". If married: both spouses sign on page 20.
File by 31 December 2026 at the latest
At Luxembourg office Y (France), Z (Germany), or X (Belgium and other countries), online via MyGuichet.lu or on paper. The online assistant rejects certain cases, for example a spouse living in another country (ACD FAQ (ouvre dans un nouvel onglet)).
Check your tax assessment notice and reassess every year
The ACD attaches the calculation of the overall rate. Conditions must be checked every year: recalculate before each declaration. To choose your procedure: tax return or annual adjustment.
Common mistakes
- Forgetting your spouse's foreign income: it must be declared, as it determines the tax rate.
- Adding together both spouses' income for the 90% test: this test is individual (only the Belgian rule looks at the household).
- Obtaining a reduced tax rate on your tax card (the ACD document indicating your tax class to your employer) without meeting the conditions: tax settlement may result in a balance due (ACD).
What could change
Bill No. 8676, introduced on 6 January 2026, provides for a single tax class in 2028. It has not been passed (Finance Committee report adopted on 29 September 2026, plenary vote still to come, Chamber of Deputies) and includes a 25-year transition period for couples already in Class 2. It could change the benefit of assimilation for certain couples. Nothing changes for your 2025 and 2026 income.
Frequently asked questions
No. It is a legal fiction that only affects the Luxembourg rate (Guichet.lu). Your tax return in your home country does not change: see declaring in your country of residence.
Yes, if one of the two spouses passes the 90% or €13,000 test. The spouse's French income is not taxed in Luxembourg, but it impacts the tax rate.
Yes: conditions are evaluated each year and the request is made in the tax return (Model 100) for the relevant year. Box 403 is used to revoke a previously requested assimilation.
No, only through the tax return (Model 100). The annual adjustment (the simple reconciliation of tax withheld on your salary, Model 163) does not allow it.