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When and how to file a joint tax return?

Are you married or in a legal cohabitation?

Written by Simon

Each year, you must file a personal income tax return. This return can be filed individually or jointly.
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In Belgium, tax rules vary depending on your family situation as of January 1st of the tax year, whether you are single, married, or in a legal cohabitation.
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Understanding when and how to complete a joint tax return is essential to avoid mistakes and optimise your taxes. Here's what you should pay attention to:
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When should you file a joint tax return?

You need to distinguish between different situations:

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👫 If you are married:

  • Your marriage took place before January 1st → you will have to file a joint return.

  • Your marriage took place after January 1st → you will have to file a separate return that year, and then a joint return the following year.

🏡 If you are legal cohabitants:

  • You entered into legal cohabitation before January 1st → your return will then be joint.

  • You entered into legal cohabitation after January 1st → your return will be separate that year, and then joint the following year.

❌ De facto cohabitants

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De facto cohabitants do not need to register administratively and have no legal obligations toward each other.

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Even after several years together, a joint return is not possible.

🧭 Key takeaway:
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✅ It is your marital status on January 1st of the tax year (the year in which you file your return and are taxed) that determines whether your return will be joint or individual.
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💡 Only married couples or legal cohabitants can file a joint return.
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⏰ January 1 is a public holiday → plan your administrative procedures in advance!
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Summary table according to your situation:

💬 Most common cases:

👤 You are...

📅 Your situation on January 1st

📄 Type of declaration

✅ What you must do

  • Single

Doesn't matter

Individual

File alone.

  • Married

Married before January 1st

Joint

One single declaration for the couple.

  • Married

Married after January 1st

Separate

Each files their own declaration.

  • Legal cohabitant

Cohabitation before January 1st

Joint

You are treated as a married couple.

  • Legal cohabitant

Cohabitation after January 1st

Separate

Each files their own declaration.

  • De facto cohabitant

Doesn't matter

Individual

No joint declaration possible.

📌 Special cases:

📌 Situation

🧾 Type of declaration

ℹ️ Useful detail

  • Divorce or legal separation


Joint (year of the judgement)

Unless already separated on 1 January.

  • Divorce or legal separation

Separate (following year)

Each person files separately.

  • Death of spouse

Joint (year of death)

The declaration covers the entire year.

  • Widowed

Individual (following year)

You declare as single.

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Advantages of a joint tax return

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The marital quotient


When you file a joint tax return, you can benefit from a specific advantage: the marital quotient.
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The marital quotient is a tax mechanism that allows a married couple or legal cohabitants to transfer a portion of one spouse's income to the other in order to reduce their overall tax.
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💡 Tip: The marital quotient is especially useful when there is a significant income imbalance between the spouses.

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In a few words

If one spouse has little or no income and the other earns significantly more, up to 30% of the combined net professional income can be allocated to the spouse with little or no professional income.​

This mechanism applies:

  • If the beneficiary spouse has no income or less than 30% of the combined professional income.

  • This mechanism applies only to married couples or legal cohabitants(and not to de facto cohabitants, who have no official registration or legal obligations toward each other).

  • Automatically: as soon as you submit a joint tax return, the marital quotient is applied without the need to request it explicitly.

  • ⚠️ Note: there is an annual maximum ceiling for this transfer (€13,460 for 2025 income).

  • Even if you have received remuneration as a supporting spouse, you may still be eligible for the marital quotient, as both can be combined under certain conditions.
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    For more information on the supporting spouse status, see this article in our FAQ: Everything you need to know about the supporting spouse status.

This allows for better use of tax brackets, thus reducing the total tax payable.
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Here's a concrete example:

  • Spouse 1 earns €45,000 of taxable income, and Spouse 2 earns €0 of taxable income.
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  • Married couple + Spouse 2 earns less than 30% of the combined income → the marital quotient can be applied.

Possible transfer: 30% of Spouse 1's income, with a maximum of €13,460 (cannot transfer more than this).
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Income after transfer to the second spouse:

  • Spouse 1: €31,540 (€45,000 minus €13,460)

  • Spouse 2: €13,460 (€0 plus €13,460)

Result: The couple pays less tax because their incomes fall into lower tax brackets compared to a normal distribution.

ℹ️ Important to know with Accountable ℹ️

With Accountable, only the part of the tax return related to self-employed income (part 2) is prepared for you. The first part must be completed separately by the non-self-employed spouse.
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👉 For the joint tax return to be valid and submitted, both parts (the self-employed part and the spouse's part) must be completed and filed together in a single submission via Tax-on-web.

When completing a joint declaration, the older person's details are entered in the left-hand column and the younger person's details in the right-hand column.
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Need help? Chat with us or email support@accountable.eu, we're happy to help.

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