Form 166 in Luxembourg: the guide for married cross-border workers

Married and working across the border? Form 166 lets you adjust your tax class or withholding rate in Luxembourg. Here's everything you need to know.

Married for years. Tax rate? Never touched.

And every year, the same story : a tax bill lands on your doormat and you owe the taxman a small fortune. Why? Because your fixed tax rate is too low. Too little gets deducted each month, and at the end of the year, Luxembourg wants the difference. Makes sense on paper. Less fun in practice.

The good news : there's a fix. Form 166. Nobody's heard of it until they desperately need it. Here's everything you should know.

What is form 166?

It's a form from Luxembourg's tax administration (the ACD) that lets you adjust your fixed withholding tax rate. That's the rate your employer uses to calculate how much tax comes off your salary every month. Officially it's called Modèle 166 F, which sounds like a sports car. It isn't. It's five pages of tax admin. But a useful five pages.

Here's what it lets you do :

• Adjust or correct your fixed tax rate
• Apply for joint taxation (tax class 2)
• Apply for individual taxation (each spouse in tax class 1, flying solo)
• Opt for individual taxation with reallocation (income pooled, you pick the split, 50/50 by default)
• Undo a previous choice and go back to the default regime

Bottom line : any time you want to change how your income is taxed at source, form 166 is the one.

Who needs this?

Every married couple in Luxembourg, whether you live here or commute across the border. But this article is really for you, the cross-border workers, who want to tweak your fixed withholding rate. That's where the form gets properly fiddly.

The usual suspects :

• Your income has changed and your current rate is miles off
• Just got married and you want tax class 2 or a fixed rate for the first time
• Switching between individual and joint taxation (or back again)
• You want to reverse a previous choice and return to tax class 1

When do you need to fill it in?

Here's the best bit : whenever you like. No deadline, no special period, no rush. January, July, October. As soon as your income changes, you can request an update. The sooner you do it, the fewer surprises later.

Both spouses sign the form, send it off to the Bureau RTS, and roughly a month later you get a shiny new tax card with the corrected rate. Your employer takes it from there. Sorted.

A real-world example

Jeanne and Paul have been married since 2019 and live in France. Back then, their tax rate was set at 5%. Since then, both their salaries have gone up. Their tax rate? Still 5%.

Every year they end up owing around €5,000 in back taxes. Not ideal.

With form 166, they get their rate adjusted. A bit more tax comes off each month, but no more heart-stopping bill at the end of the year. Just peace of mind.

5 pages, 5 scenarios, 0 enthusiasm

Let's be honest : form 166 is not anyone's idea of a good time. Five pages. Five different scenarios to unpick. Tick the wrong box and you've got the wrong tax rate on your payslip for months.

Then there's page 4, where you fill in your financial details : worldwide income, Luxembourg income, the lot. Get a number wrong here and it's your take-home pay that takes the hit.

Thrilling Saturday night activity? Thought not.

Let taxx.lu sort it

Five pages of tax form versus your evening. Not a fair fight. For €59, taxx.lu handles the whole thing :

• We fill in form 166 with the correct financial data to calculate your rate
• We check your assimilation conditions
• You get a ready-to-sign form : just post it and you're done

No headaches. No mistakes.

All we need from you :

• The last 3 payslips from each spouse
• A copy of the marriage certificate

That's literally it.