Before you start

A French tax number, issued after your first declaration
Records of worldwide income, including any Belgian salary and withholding
A record of days worked in France versus in Belgium, if you telework
Credentials for impots.gouv.fr, once you have a number

Step-by-step

1
Establish that you are a French tax resident, whoever pays you
Your home and your family being in Lille makes you resident in France for tax purposes, taxable on worldwide income, regardless of which country your employer is in. Everything else is about how double taxation is relieved, not about whether you are in the French system.
OnlineWho: You
2
Do not assume the frontier-worker regime applies to you
The special regime that taxed a French resident's Belgian salary in France was closed to new entrants from 1 January 2012 and survives only for people continuously in post since before then, until the end of 2033. If you start a Belgian job now, Belgium taxes the salary.
OnlineWho: You
3
Keep a day-by-day record of where you physically worked
Days worked from home in France are taxable in France; days worked in Belgium are taxable in Belgium, apportioned. There is no agreed tolerance between the two countries, so this is not an approximation you can make at the end of the year. A simple calendar kept as you go is the difference between a clean declaration and a reconstruction.
OnlineWho: YouContinuously
4
Ask your employer to apply for the social security derogation if you telework
Under the European framework agreement in force since July 2023, which both France and Belgium have signed, an employer can apply for a teleworking cross-border employee to remain in the employer state's social security as long as home working stays below 50% of working time. It is an application with an A1 certificate at the end of it, not an automatic entitlement, and only the employer can make it.
Via employerWho: Your employerBefore the telework pattern starts
5
File the French declaration and report the Belgian income
You declare the Belgian salary in France on the appropriate schedule even though Belgium has taxed it. France relieves the double taxation, but the income is taken into account in determining the rate applied to the rest of your household's income. Skipping the declaration because Belgium already took the tax is a common and expensive error.
OnlineWho: YouSpring annually
6
Declare every foreign account, including the Belgian one
French tax residents must list all foreign bank accounts, life insurance contracts and digital asset accounts annually, whether or not they generated income. The Belgian salary account is exactly the kind of account people forget because it feels domestic.
OnlineWho: You

Documents you’ll need

French tax number, once issued
Belgian payslips and the annual Belgian tax documents
A calendar of days worked in each country
A1 certificate, if the social security derogation was granted
Marriage or PACS documentation, for household parts

Things most newcomers don’t know

The France–Belgium frontier-worker regime is closed, and almost everything written about it online is out of date.

The regime that let a French resident's Belgian salary be taxed in France was shut to new entrants from 1 January 2012 by the 2008 amendment to the 1964 treaty, and continues only for people in continuous employment since before that date, ending on 31 December 2033. Blogs, forum posts and even some employer HR packs still describe it as though it were available. Anyone taking a Belgian job today is taxed in Belgium, at Belgian rates including the municipal surcharge, and should model the net from that basis.

Source: impots.gouv.fr / avenant du 12 décembre 2008

There is no telework tolerance with Belgium — from the first day, home days are taxable in France.

France agreed a tolerance of a set number of home-working days with Luxembourg and with some other neighbours. It has not with Belgium. Employment income is taxable where the work is physically performed, so a Lille resident working two days a week from home splits their salary across two tax systems pro rata, every year, with a Belgian return and a French one. People assume a Luxembourg-style threshold exists because they have read about one.

Source: Frontaliers Grand Est / convention franco-belge de 1964

The 50% figure is a social security rule and has nothing to do with tax.

The European framework agreement in force since July 2023, signed by both France and Belgium, lets an employer apply for a cross-border teleworker to stay in the employer state's social security provided home working stays under half of working time. That threshold governs which country's contributions, pension and sickness rights apply. It says nothing about income tax, where the split is pro rata from day one. Conflating the two is the single most common cross-border planning error in this city.

Source: Frontaliers Grand Est / EU framework agreement

The replacement France–Belgium treaty has been signed since 2021 and still is not in application.

A new convention was signed on 9 November 2021 to replace the 1964 text, and its entry into application has slipped repeatedly, with particular controversy over the treatment of French residents working for Belgian public employers. Until it applies, the 1964 rules govern. This is a genuine live reform rather than a settled position, so any advice you receive should be dated — and anything you read from before the last budget cycle should be checked.

Source: Sénat / impots.gouv.fr

Common mistakes to avoid

Assuming the frontier-worker regime is available to someone starting a Belgian job now.
Applying a Luxembourg-style telework tolerance to Belgium, where none exists.
Treating the 50% social security threshold as though it governed income tax.
Not declaring Belgian income in France because Belgium has already taxed it.
Failing to declare the Belgian bank account on the French foreign accounts form.

Some of this may be out of date. Spotted something inaccurate? Help us keep it right for the next newcomer.