Can you only index rent once a year?

Q: Can you only index rent once a year?

Yes, Article 1728bis of the Civil Code provides that indexation can only be requested once per year, on the anniversary date of the lease entry into force. The landlord cannot index quarterly or semi-annually, even if the lease contains such a clause. Any clause providing for more frequent indexation is deemed unwritten.

5 min read Verified on May 13, 2026 By Edouard Hennin

Quick answer

Yes, indexation can only be requested once per year, on the anniversary date of the lease entry into force. The landlord cannot index more frequently, even if the health index rises sharply.

Can you only index rent once a year?

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Indexation is limited to once per year

Quick answer

Yes, the rent can only be indexed once per year, on the anniversary date of the lease’s entry into force. The landlord cannot index more often, even if the health index rises significantly. Any lease clause providing for more frequent indexation is void.

Article 1728bis is clear on this point:

Indexation is annual — once every 12 months
The trigger date is the anniversary of entry into force
No quarterly, semi-annual or other frequency is permitted
A clause providing otherwise is deemed unwritten (legally void)

Invalid frequency clauses

These clauses in a lease are legally void:

“The rent will be indexed every 6 months”
“The rent will be adjusted quarterly based on the health index”
“The landlord reserves the right to index at any time”

If your landlord attempts to apply indexation more than once per year, you can refuse and contest the additional increase.

Practical tip

If your lease contains an invalid frequency clause, the clause is simply ignored but the rest of the lease remains valid. You still have the right to one annual indexation on the correct date.

Why annual indexation only

The annual limitation serves several purposes:

Tenant protection: prevents excessive or unpredictable rent increases
Predictability: both parties know when the next adjustment will occur
Administrative simplicity: one calculation per year is manageable for both parties
Inflation smoothing: annual calculations smooth out short-term index fluctuations

Even during periods of high inflation (as in 2022-2023), the annual rule applies. The landlord benefits from the full year’s inflation at the anniversary date.

Regional specifics

Brussels-Capital Region

The Ordinance of 27 July 2017 confirms the annual indexation rule without exception.

Wallonia

The Decree of 15 March 2018 applies the same annual limitation.

Flanders

The Flemish Housing Rental Decree of 9 November 2018 confirms annual indexation only.

Legal basis

Article 1728bis of the Belgian Civil Code. The annual frequency is a mandatory rule that cannot be modified by contract.