Forgot to index? These are your rights as a landlord

It happens to many landlords: the anniversary of the lease passes unnoticed and the rent was not indexed in time. The good news is that you can still apply indexation retroactively. The bad news: only up to 3 months back.

The 3-month rule

Belgian law allows landlords to apply rent indexation with retroactive effect, but with a strict limit: maximum 3 months.¹ Specifically: if you send the indexation letter today, the new rent can take effect at the earliest on the first day of the month that began 3 months ago.

Everything older than 3 months is permanently lost. You cannot recover missed indexations from previous years.

What is lost, what can you recover?

Missed indexations older than 3 months are permanently lost. You cannot recover these periods.

The last 3 months can still be recovered retroactively. Act now to avoid losing these too.

From next month onwards, the new indexed rent applies. Make sure you send the letter in time.

⚠️ Watch out if you have not indexed for a long time Suppose you have not indexed for 2 years. You can still index today, but only using the index from the anniversary date that falls within the past 3 months. Concretely: of the 24 months you have not indexed, 21 are permanently lost. Only the last 3 months can still be recovered.

Multiple years forgotten: what now?

If you have not indexed for several years, you apply the indexation once using the health index of the anniversary date that falls within the past 3 months. You do not apply multiple years of indexation cumulatively — that is not permitted.

Practical example: your lease started on 1 April 2021. You forgot to index in 2022, 2023, 2024 and 2025. Today it is March 2026. The most recent anniversary within 3 months is 1 January 2026 (anniversary of April 2021 = April, but last anniversary within 3 months = January 2026). You use the health index for December 2025 and the new rent takes effect on 1 January 2026.

📌 Communicate clearly about the outstanding amount If you apply indexation retroactively, the tenant owes the difference for the past months. Communicate this clearly in your letter and possibly agree on a payment plan to avoid disputes.

How do you proceed?

Determine the earliest effective date
Go back maximum 3 months from today. The indexation can take effect from the 1st of that month.
Look up the correct index
Use the health index of the month before the anniversary that falls within those 3 months. Our rent tool calculates this automatically.
Calculate the new rent
Base rent × new health index ÷ base index. If applicable, apply the EPC correction factor.
Send a written notification
Include the calculation, effective date and legal basis (Art. 1728bis Civil Code).

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Legal sources

The 3-month retroactive limit follows from case law and the prevailing interpretation of Art. 1728bis §3 of the Civil Code (Law of 20 February 1991).