Cross-border worker in Luxembourg: how can you benefit from tax assimilation?

What is tax assimilation in Luxembourg?
 

Tax assimilation is a scheme put in place by the Direct Tax Administration (ACD). It allows you, as a non-resident worker, to be taxed like a Luxembourg tax resident.
 

This status is not granted automatically: you must apply for it, and your case is assessed according to specific criteria set by the administration. It mainly applies to withholding tax on salaries.
 

To qualify, you must meet one of these two conditions:

• Earn at least 90% of your professional income for the year in Luxembourg.
• For Belgian residents, it is possible to apply for assimilation only if more than 50% of the household’s professional income is taxable in Luxembourg.
• Have worldwide income received abroad of less than €13,000.
   

For married couples or couples in a civil partnership, at least one of the partners must meet one of the three conditions for the couple to be able to apply for assimilation.
 

Who can apply for tax assimilation?
 

This status is primarily intended for cross-border workers from France, Belgium or Germany who earn a salary in Luxembourg and are already subject to withholding tax. It also concerns non-residents who have strong economic ties with the country.
 

The Direct Tax Administration reviews your professional circumstances, how long you have been present in Luxembourg and the breakdown of your household’s worldwide income. If your application is accepted, you benefit from the same tax scales as a resident for the calculation of your income tax.
 

What criteria must be met to be treated as a Luxembourg resident?
 

To approve your application, the Direct Tax Administration relies on precise accounting criteria. Even if your home or centre of interest is not in Luxembourg, it is the concentration of your economic income in the country that justifies this tax residence treatment.
 

Your application is examined as a whole. Each supporting document must show that your situation complies with the cross-border taxation thresholds laid down by law, based on the official data of the tax authorities.
 

How do you apply for tax assimilation?
 

You must submit an official file directly to the competent ACD tax office. For your application to be processed efficiently, attach the completed official form as well as:
 

• Your Luxembourg employment contract and all your payslips for the year concerned.
• Your residence certificates and household composition certificates, issued by your country of origin.
• Supporting documents for all income received outside Luxembourg (such as your tax assessment notice in your country of residence).
   

Processing times vary depending on the time of year. It is therefore better to submit a complete file from the outset, to avoid lengthy back-and-forth requests for additional information.
 

The application must be renewed each year when filing your income tax return using form 100. On this form, you must tick the section “Assimilation of a non-resident to resident” and declare all your income (Luxembourg and foreign).
 

What tax advantages does assimilation to Luxembourg resident status offer?
 

The main benefit of this status is that it can optimise your household’s tax position. Assimilation allows you to be taxed as if you were a Luxembourg resident. 
 

The aim: to guarantee equal treatment between residents and cross-border workers in comparable situations. Without this assimilation, you are taxed only on your Luxembourg income, in class 1, without the possibility of deducting your expenses. With assimilation, you have access to the same deductions as a resident, such as:    
 

• Interest on loans linked to your main residence or consumer loans.
• Your insurance premiums (car, home, protection insurance).
• Payments into a retirement savings or home savings policy.
• Your extraordinary expenses (childcare costs, costs of assistance for a dependent parent, unreimbursed health expenses, etc.)
   

It is an effective tool for limiting double taxation and better balancing your salary and non-salary income.
 

Please note, however, that assimilation is not always advantageous: we recommend comparing taxation each year with and without assimilation before filing your return. 
 

Tax assimilation and tax residence: what is the difference?
 

These two concepts are clearly distinct:
 

Tax resident in Luxembourg: You have your main home or you stay habitually and for an extended period in the country (more than six consecutive months in the year).
 

Assimilated taxpayer: You legally remain a non-resident – a cross-border worker living abroad – but you benefit, notwithstanding this and depending on your financial situation, from tax treatment modelled on that of residents.
 

What mistakes should you avoid in your tax assimilation application?
 

To give yourself the best chance with the Direct Tax Administration, keep these tips in mind:
 

• Avoid incomplete applications: Never send documents that are crossed out, illegible or missing.
• Be transparent: Present your payslips and your foreign income tax returns clearly and in a structured way.
• Meet the deadlines: If the administration asks you for additional supporting documents, reply as quickly as possible to avoid a rejection on purely administrative grounds.