Frontalier taxes in Luxembourg: the 90% rule and the remote-work day limits explained

Luxembourg runs on cross-border labour. On any given workday, roughly half of the country's salaried workforce crosses a border from France, Belgium or Germany. These frontaliers (cross-border workers) face a tax and social-security setup that is often misunderstood — partly because three different rulebooks overlap, each with its own threshold.

Taxed at source, in class 1 by default

Frontaliers who work physically in Luxembourg are, under the double-tax treaties with France, Belgium and Germany, taxed in Luxembourg on their Luxembourg-source employment income. Tax is withheld at source by the employer. By default, a non-resident is slotted into tax class 1, regardless of marital status — which generally means a higher effective rate than the class 2 available to many residents.

The sting is that class 1 ignores the household: a married non-resident commuter is taxed as a single person unless they take action.

The 90% rule and resident assimilation

The fix is assimilation to a resident taxpayer. A non-resident can request to be treated like a Luxembourg resident — gaining access to tax class 2 (joint taxation for couples) and the full set of resident deductions, allowances and tax credits — if they meet one of these conditions:

• At least 90% of their worldwide income is taxable in Luxembourg; or
• Their net income not taxed in Luxembourg is below €13,000 per year.

Residents of Belgium have an extra route: assimilation is also possible if 50% of the household's professional income is taxable in Luxembourg.

A practical detail: when computing the 90% quota, days worked abroad and exempt under a treaty still count as Luxembourg income for up to 50 days. Assimilation is assessed every year, and foreign income is still used to set the applicable tax rate even though it is not taxed in Luxembourg. The request is made via guichet.public.lu or on the tax return (form 100).

The 34-day remote-work tolerance (this is a tax rule)

Working from home changes where a day is taxed. Under the treaties, a day physically worked at home in France, Belgium or Germany would normally be taxable there, not in Luxembourg. To spare commuters from splitting their salary across two tax systems, each treaty grants a tolerance threshold of days that stay taxable only in Luxembourg:

• France: 34 days per year (treaty amendment applicable from 2023).
• Belgium: 34 days per year (applicable from 2022).
• Germany: 34 days per year (from 2024, up from the previous 19 days).

Cross the threshold and the entire block of home-working days — not just the excess — becomes taxable in the country of residence, with all the filing complications that brings.

Social security: a different 49% rule

Here is the trap: the 34 days is a tax limit, not a social-security limit. Social-security affiliation is governed by EU Regulation 883/2004. The default rule is that working 25% or more of your time in your country of residence shifts your social security there. But the EU multilateral framework agreement on cross-border telework, in force since 1 July 2023, lets employees telework up to 49% (less than 50%) of their time from home and stay insured in Luxembourg — provided both the employer's and the worker's countries have signed (France, Belgium and Germany all have). It is not automatic: an application and an A1 certificate are required.

So a frontalier could comfortably stay within social security in Luxembourg at, say, two days of home-working a week, yet blow past the 34-day tax tolerance — and owe tax at home. Tracking both counters separately is essential.

Last reviewed: June 2026. Day thresholds and the 90% rule are current as of this date; treaty terms and the teleworking framework agreement can change — always confirm on guichet.public.lu and the Administration des contributions directes.