I worked remotely from Spain for a Belgian employer before moving to Belgium in 2025: do I owe Belgian tax for assessment year 2026?

If you were resident in Spain and physically performed your employment there before moving to Belgium, Belgium will generally not be entitled to tax that pre-move salary merely because the employer was Belgian. A qualifying Belgian doctoral scholarship may also be exempt. A large pre-filled Belgian tax balance can therefore be misleading, but the correct treatment depends on the exact residence facts, income classification and return codes.

The figures below are illustrative and relate to income year 2025, assessment year 2026.

When does Belgian tax residence start after moving to Belgium?

For someone who lived and worked abroad for most of 2025 and moved to Belgium only near the end of the year, the first question is not the employer’s location but when Belgian tax residence actually began.

Registration in the Belgian National Register creates a legal presumption of Belgian tax residence. In a typical late-year immigration case, the Belgian resident taxable period may therefore cover only the final part of 2025 rather than the full calendar year.

For example, if the taxpayer became Belgian resident only during December 2025, the Belgian resident taxable period could be limited to that month. Tax-free allowances and certain benefits may then be prorated because the taxable period is shorter than a full calendar year.

The foreign country may nevertheless apply its own residence rules over the full calendar year. If both Belgium and Spain consider the taxpayer resident during an overlapping period, the Belgium-Spain double tax convention must be used to resolve the conflict.

Is salary from a Belgian employer taxable in Belgium if the work was performed from Spain?

Not necessarily.

Article 15 of the Belgium-Spain double tax convention starts from the principle that employment income received by a resident of one State is taxable in that State unless the employment is exercised in the other State.

For employment income, the decisive factor is generally where the employee was physically present while performing the work.

Consider an illustrative case:

• the taxpayer lived in Spain during most of 2025;

• the taxpayer was Spanish tax resident during that period;

• all work was performed remotely from a home in Spain;

• the employer happened to be Belgian;

• gross salary before the move was about €18,000.

On those facts, the Belgian location of the employer does not by itself give Belgium the right to tax the salary. The treaty analysis points instead toward taxation in Spain for the period during which the employment was physically exercised there.

This also means that a Belgian tax calculation that automatically treats the entire annual Belgian salary fiche as Belgian taxable income may produce an incorrect result.

If Belgian professional withholding tax was deducted from income that Belgium ultimately has no right to tax, a refund may in principle be possible. The usefulness of pursuing that refund depends on the amount actually withheld and the individual file.

A Spanish certificate of tax residence for 2025, together with evidence of residence and the Spanish tax return, can be particularly important in supporting the treaty position.

What about Belgian social security deducted while working remotely from Spain?

Tax and social security are separate questions.

A person working fully remotely from Spain may, under European social security coordination rules, fall under the Spanish social security system even though the employer is Belgian.

If Belgian social security contributions were nevertheless deducted, this may raise a separate compliance issue. It does not automatically change which country has the right to tax the salary.

Because social security corrections can affect both employer and employee obligations, this issue should be analysed separately from the income tax return.

Is a Belgian doctoral scholarship taxable?

A genuine doctoral or research scholarship granted by a Belgian university can be exempt from Belgian personal income tax when the applicable conditions are satisfied.

The analysis supplied for this case identifies three important elements:

• it must be a genuine study and research bursary;

• there must be no employment contract;

• the researcher must devote herself or himself to the doctoral research.

Social security contributions may still be deducted from an exempt scholarship.

In an illustrative case where a doctoral bursary is about €3,000 gross per month, the fact that social security is deducted but no professional withholding tax appears on the payment statement may be consistent with the university treating the payment as an exempt scholarship.

That does not mean that every payment called a “scholarship” is automatically exempt. The contractual structure and the conditions of the bursary must be checked.

Why can Tax-on-web show a balance of around €5,000 when little or no Belgian tax is actually due?

A pre-filled return is based on information that the Belgian administration has received. It does not necessarily understand the taxpayer’s cross-border residence history.

A common problem in a migration year is that Tax-on-web imports a full-year fiche 281.10 and initially treats the salary as Belgian taxable professional income.

Suppose the system includes approximately €18,000 of salary as Belgian taxable income while only a small amount of professional withholding tax was deducted. Progressive Belgian tax rates and the applicable communal surcharge can then generate a simulated balance of several thousand euros.

A balance of around €5,000 can therefore be a calculation based on an incorrect taxable base rather than the taxpayer’s real liability.

In a correctly analysed case, the return may instead need to reflect that:

• salary earned before Belgian residence was exercised in Spain and falls under the treaty;

• income falling outside the Belgian resident taxable period must be treated correctly;

• a qualifying doctoral scholarship must not be taxed as ordinary professional income.

The precise return codes and the way the foreign income is reported matter. This is why simply accepting the pre-filled return can be risky in a cross-border migration year.

What documents help support the correction?

For a typical Belgium-Spain migration file, useful supporting evidence can include:

• the Spanish certificate of tax residence for 2025;

• Spanish registration or residence documents;

• the Spanish income tax return;

• Belgian employer payslips and annual tax fiches;

• university scholarship documents and payment statements;

• Belgian registration documents;

• evidence showing where the employment was physically performed.

Tax-on-web allows supporting documents to be attached. A concise explanatory annex can also set out the residence timeline, treaty position and scholarship treatment.

A corrected return may trigger a request for information from the tax administration. That is not unusual in a cross-border case.

What are the Belgian filing deadlines for assessment year 2026?

For assessment year 2026, income year 2025, the source analysis identifies the following deadlines:

• 30 June 2026 for paper returns;

• 15 July 2026 for standard online returns via MyMinfin;

• 16 October 2026 for online returns containing certain specific income, expressly including foreign professional income.

The 16 October 2026 deadline is automatically available when the return meets the relevant conditions and is indicated in the final steps of Tax-on-web.

There is an important technical nuance in a migration case.

If foreign professional income is reported in the return together with the applicable treaty treatment, the return may fall within the category eligible for the extended October deadline.

If the return is instead prepared strictly as a short-period Belgian resident return containing only income relating to the Belgian residence period, the automatic availability of the extended deadline may be less clear.

Both approaches can potentially lead to a similar substantive tax result while producing a different answer on the filing deadline. The correct method therefore depends on the actual facts and how the return must technically be completed.

What happens if the return is filed late?

According to the source analysis, Belgian law allows two main sanctions for late filing:

• an administrative fine of €50 to €1,250, with a first infringement normally starting at €50;

• a tax increase ranging from 10% to 200%.

The tax increase is percentage-based. Where the correctly established Belgian tax is zero or close to zero, the practical amount generated by a percentage increase may therefore also be zero or limited.

Force majeure can also be decisive.

Documented serious medical circumstances or other circumstances genuinely preventing timely filing can support a request for a sanction to be waived. Good faith and the fact that it is a first infringement are also relevant factors.

The analysis further notes that Belgian administrative practice following a Constitutional Court ruling recognises that a first offence committed in good faith should not automatically result in a 10% tax increase.

Whether a particular set of circumstances amounts to force majeure must nevertheless be assessed individually and supported by evidence.

What if no return is filed at all?

Doing nothing is materially riskier than correcting and filing the return.

After the applicable deadline, the administration can in certain circumstances establish an ex officio assessment using the information already available to it.

In a cross-border case, that information may include the same pre-filled salary figures that produced the incorrect simulation.

An ex officio assessment can also shift the burden of proof toward the taxpayer and can involve a longer assessment period.

If an incorrect assessment or sanction is issued, the source analysis notes that an administrative objection can generally be lodged within one year of receiving the assessment notice.

Do foreign bank accounts need to be declared after becoming Belgian resident?

Yes. Once a person becomes a Belgian tax resident, foreign bank accounts can create additional reporting obligations.

The taxpayer must in particular:

• indicate the existence of a foreign account in the Belgian annual tax return; and

• register the foreign account with the Central Point of Contact of the National Bank of Belgium.

The source analysis states that the account should be registered no later than when filing the return in which it is mentioned.

This reporting obligation is separate from whether the money held in the foreign account is itself taxable.

Frequently asked questions

My employer was Belgian. Does that automatically mean my salary is taxable in Belgium?

No. For employment income under the Belgium-Spain treaty, where the work was physically performed is a central factor. A Belgian employer does not automatically make salary earned while working from Spain taxable in Belgium.

Why is my Belgian pre-filled return showing thousands of euros due?

Tax-on-web may have imported a full-year Belgian salary fiche without correctly reflecting a move to Belgium during the year or the treaty treatment of foreign work. The simulation can therefore be based on income that Belgium ultimately has no right to tax.

Is a PhD scholarship in Belgium always tax-free?

No. The exemption depends on the scholarship satisfying the required conditions, including its genuine research purpose and the absence of an employment relationship. The contractual documents must be checked.

Can I use the 16 October 2026 filing deadline because I had foreign income?

Possibly. The extended deadline applies to returns containing certain categories of income, including foreign professional income. In a migration-year case, whether it applies can depend on how the foreign income must technically be reported.

What is the fine for filing a Belgian tax return late?

For assessment year 2026, the source analysis refers to an administrative fine ranging from €50 to €1,250 and possible tax increases of 10% to 200%. Force majeure, good faith and the actual amount of tax due can materially affect the outcome.

Should I simply delete the pre-filled Belgian salary from Tax-on-web?

Not without analysing the residence period, treaty treatment and correct reporting method. Cross-border income may need to be removed, reclassified, reported with an exemption claim or explained in an annex depending on the individual situation.

Sources

1. Belgium and Spain double tax convention of 14 June 1995, assent law with full text: https://etaamb.openjustice.be/fr/loi-du-10-aout-1998_n2003015145.html

2. FOD Financien, press file on the 2026 personal income tax return campaign and deadlines: https://fin.belgium.be/sites/default/files/media/documents/persconferentie-aangifte-2026.pdf

This article explains a general tax framework and does not constitute a personalised tax opinion. Tax rules, administrative practice and filing procedures can change from one year to another. The correct treatment depends on the taxpayer's exact residence history, employment circumstances, contractual documents, income types and tax return data.

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