In Belgium, not all taxpayers have the same filing deadline. For assessment year 2026, covering income received in 2025, some individuals can file their return online until 16 October 2026. This extension concerns what is commonly called a complex tax return.
The tax authorities use a more precise description: returns containing certain specific types of income. The extended deadline does not automatically apply simply because your tax affairs seem complicated.
Who qualifies for the 16 October 2026 deadline?
The extended deadline covers, in particular, taxpayers declaring:
- business profits or income from self-employed professional activities;
- company directors’ remuneration;
- remuneration of an assisting spouse or legally cohabiting partner;
- foreign professional income.
It may also apply when a person declares one of the following for the first time during this filing campaign:
- real estate located abroad;
- maintenance payments received from abroad or paid to someone living abroad;
- a loan taken out abroad;
- a legal arrangement;
- application of the special tax regime for inbound taxpayers or researchers.
FPS Finance also includes taxpayers who declared a legal arrangement in the previous year. These categories appear in its official conditions for specific income (in French).
Receiving a Belgian salary, claiming a tax reduction or correcting pre-filled information does not necessarily qualify you for the extension.
Why is it called a complex tax return?
These returns generally require more checks and supporting documents.
A self-employed person must determine their taxable result, distinguish business expenses from private expenses and check their social security contributions. A company director may need to report remuneration, benefits in kind and other income connected with their company.
Foreign income requires additional analysis:
- identifying which country has the right to tax the income;
- applying any relevant double taxation treaty;
- converting amounts into euros;
- distinguishing between exemption and taxation in Belgium;
- providing information about foreign assets, accounts or contracts.
The extra time allows you to gather this information, but does not reduce your reporting obligations.
The 16 October deadline is for online returns only
The extended deadline applies only to returns filed electronically through MyMinfin – Tax-on-web.
Paper returns follow a different timetable. For assessment year 2026, the paper filing deadline was 30 June 2026. The standard online deadline was 19 July 2026, while returns containing the specified income can be filed until 16 October 2026. The FPS Finance filing calendar (in French) distinguishes these dates.
It is therefore essential to distinguish between:
- the actual complexity of your circumstances;
- the filing method you choose;
- the official criteria for the extended deadline.
The applicable date is normally shown in MyMinfin. This timetable concerns personal income tax; non-resident income tax returns follow a separate calendar.
What if MyMinfin shows an earlier deadline?
MyMinfin may initially show the standard deadline even though your return includes specific income.
You can indicate this when completing your return, at the final step before submission. Tick the relevant box and, where necessary, attach supporting documents for the income or expenses concerned. The extension is granted automatically if the conditions are met.
Check this early enough to prepare your file and the necessary supporting evidence.
Check any simplified tax return proposal carefully
Receiving a simplified tax return proposal does not mean that all the information is correct or complete.
In particular, check the proposal if you received income not included by the authorities or hold foreign assets that must be reported.
Where a simplified proposal needs to be replaced by a return containing specific income, FPS Finance requires advance notification to obtain the 16 October deadline. For the 2026 filing campaign, it recommended making contact preferably before 15 July 2026.
That date has passed, so contact the competent tax office promptly if you discover an omission in September. Do not assume that you can still amend the proposal directly online or that the extension has been granted.
Why should you avoid leaving preparation until the last minute?
The extra time should help you prepare an accurate return, rather than postpone the work.
You may need to collect numerous documents:
- accounts or business results from self-employment;
- company director remuneration statements;
- social security contribution certificates;
- supporting documents for business expenses;
- records of foreign professional income;
- foreign tax assessment notices;
- purchase deeds and cadastral information for foreign property;
- loan agreements or documents relating to a legal arrangement.
Filing late may also extend the period during which the authorities can assess tax. For assessment year 2026, the ordinary period is generally three years, while it may reach four years for a late or missing return or certain complex situations.
The concept of a complex return used for these audit periods should not be confused with the criteria for the 16 October filing deadline.
Preparing your file in September leaves time to obtain missing documents, check applicable tax treaties and correct inconsistencies.
How can you prepare a reliable complex tax return?
A complex return should be approached as a complete file rather than simply a form.
In particular, you should:
- check the deadline shown in MyMinfin;
- identify all Belgian and foreign income;
- reconcile amounts with the available statements and evidence;
- analyse deductible business expenses;
- check reporting obligations for foreign assets and contracts;
- attach the required supporting documents;
- retain the records used to complete the return.
FPS Finance requires taxpayers to keep documents needed to calculate their tax for seven years. See its guidance on tax return checks and record retention (in French).
At Tax Law International, we assist self-employed individuals, company directors, expatriates and taxpayers with foreign income in preparing complex tax returns.
Is your return due by 16 October 2026?
An initial review can confirm the applicable deadline, identify the documents needed and ensure that professional or international income is handled correctly.
Explore our services for personal income tax for residents and expatriates and foreign residents, or contact Tax Law International to prepare your return.
