The Personal Income Tax return form is coming soon, filing deadline 30 June 2026 (paper version) or 15 July 2026 (electronic version), unless extended. Owner-landlords who paid mortgage interest in 2025 are asking an urgent question: how do I preserve my rights if the deduction is removed? The answer: file a written annex with the return. Step-by-step guide.
The tax context: what changed on 17 March 2026
For income year 2024, there was a specific section in the tax return: codes 1146-18 and 2146-85. These sections allowed taxpayers with interest payments on a mortgage loan to declare them and obtain a reduction of their taxable base under Article 14, 1° of the Income Tax Code 1992 (ITC 1992).
The Royal Decree of 17 March 2026 removed these tax deduction codes. The practical consequence: owners who declare property income while having outstanding loans to acquire or preserve such taxable income no longer have a section in which to deduct that interest.
Why this removal is legally fragile
Several tax lawyers point out that this abrupt removal, without a transitional regime for ongoing loan contracts, is legally contestable. An appeal has been brought before the Constitutional Court on two main grounds:
- Excessive infringement of taxpayers' legitimate expectations, without a compelling reason of general interest justifying the absence of a transitional regime for ongoing contracts, which would constitute a violation of Articles 10, 11 and 172 of the Constitution
- Infringement of the principle of "fair taxation"
The appeal is under review and will not be decided before your 2026 return is filed. The question for the taxpayer becomes: what to do in the meantime?
The answer: file a conservatory annex declaration
To avoid losing your rights if the Constitutional Court were to strike down the removal, you must attach a written declaration to your PIT for tax year 2026 claiming, on a precautionary basis, the deduction of the mortgage interest paid in 2025.
This step pursues two objectives:
- Preserve the evidence that you expressly claim this deduction (rather than having "forgotten" it)
- Position yourself to benefit from any favourable ruling by the Constitutional Court, which could impose a regularisation mechanism
Without this annex, the tax administration could argue that you accepted the removal without reservation.
Sample declaration to file
The wording proposed by several Belgian tax lawyers, including Olivier de Clippele, tax lawyer, takes the following form:
Declaration to attach to the tax return (tax year 2026, income year 2025)
In addition to the PIT return form for tax year 2026, income year 2025, the declarants reserve all rights concerning the interest they actually paid during 2025 on debts contracted to acquire or preserve property income, such interest having been reportable under codes 1146-18 and 2146-85 of the previous PIT return.
This reservation is justified by the fact that reading the Belgian Official Gazette of 30 December 2025 does not allow one to conclude that the second paragraph of Article 14 ITC 1992 has been removed, and by the fact that an appeal is pending before the Constitutional Court for the following reasons:
1° excessive infringement of taxpayers' legitimate expectations, without a compelling reason of general interest justifying the absence of a transitional regime for ongoing contracts (violation of Articles 10, 11 and 172 of the Constitution).
2° infringement of the principle of "fair taxation".
The declarants claim, on a precautionary basis, the deduction of the interest referred to in Article 14, 1° ITC 1992 for an amount of:
- .......... euros (former code 1146-18)
- .......... euros (former code 2146-85)
Declarants:
- Surname, first name, national number: ..........
- Surname, first name, national number: ..........
Date + signature(s)
This declaration is then attached to the Personal Income Tax (PIT) return.
The supplementary mention to add on the PIT form
To ensure that the annex is properly linked to the main form, you should add to page 13 of the PIT return form:
"Number of sheets attached: .......... "
With, in addition:
"including a supplementary declaration for the deduction of interest borne in 2025 (tax year 2026) and referred to in Article 14, 1° ITC 1992, which is not removed"
This double mention explicitly establishes the link between the form and the annex, and prevents the administration from treating the annex as a detached document.
Always keep proof of filing
This step will have no value if you cannot prove you carried it out. Three precautions:
- For a paper return: send by registered post with acknowledgement of receipt. Keep the receipt and a complete photocopy of the file sent.
- For an electronic return via Tax-on-web: attach the annex in PDF format when filing, and keep the filing acknowledgement that the platform generates automatically.
- In all cases, archive a copy of the signed annex with your personal tax file.
Why this proof is crucial
If the Constitutional Court were to annul the removal, or even impose a transitional mechanism, taxpayers who have previously claimed their rights in writing will be in a strong position to assert the deduction. Others might have to bring an individual procedure, the outcome of which is much more uncertain.
For which taxpayers is this step relevant?
This annex mainly concerns owners who, in 2025, have:
- Paid mortgage interest linked to the acquisition or preservation of a property generating taxable rental or property income
- Reported this interest, or could have reported it, under codes 1146-18 and 2146-85 of their PIT return for tax year 2025
- Still had ongoing loans for these properties in 2025
If you are in this situation, the step is free (apart from any registered-post fees) and defensive: it does not expose you to any tax risk, but it preserves your rights in the favourable scenario.
If you are not sure of your situation, jointly held properties, dismembered ownership, complex structures, do not hesitate to consult your usual tax adviser. One hour of consultation can represent a considerable return on investment if the Constitutional Court strikes down the removal.
And then?
The Constitutional Court's decision is expected in the coming months. Three possible scenarios:
- The Court annuls the removal, taxpayers who attached the annex will be able to assert their rights as part of a regularisation mechanism
- The Court imposes a transitional regime for ongoing contracts, here too, the annex serves as proof of claim
- The Court rejects the appeal, the annex will have no positive consequence, but it will only have cost the price of a stamp
In two out of three cases, the step is a winner. In the third, it is neutral. This is what is called a tax bet with no downside.
For further reading, see also our article on the removal of the mortgage interest deduction in Belgium, the origin of this tax matter.
Streamline the monitoring of your tax obligations with Seido, centralised documents (leases, returns, supporting documents), reminders for deadlines and structured communication with your accountant.