Is rent indexation automatic?

Rent indexation is never automatic

Article 1728bis of the Civil Code is clear: indexation “can be requested” (peut etre demandee). This wording implies an active step by the landlord. Without a written request:

• The tenant can continue paying the non-indexed rent
• The tenant is not in default
• The landlord cannot later claim the difference retroactively (beyond 3 months in most regions)

How to properly request indexation

To apply indexation correctly:

1. Calculate the new rent using the indexation formula
2. Write a letter or email to the tenant with the full calculation details
3. Send it on or after the anniversary date of the lease entry into force
4. Keep proof of sending (email receipt, registered mail tracking)
5. Apply the new rent from the month following the request

The request must include the base rent, both health indices (starting and new), and the resulting indexed rent.

Common misconceptions

“My lease says indexation is automatic”: even with such a clause, a written request is legally required. The clause simply confirms the right to index, it does not remove the notification obligation.

“My tenant should know the rent goes up”: the law places the burden on the landlord to inform and request. The tenant has no obligation to anticipate indexation.

“I can deduct the difference from the deposit”: this is not permitted. The deposit secures rental damage and unpaid rent, not missed indexation that was never formally requested.

Regional specifics

Brussels-Capital Region

The Ordinance of 27 July 2017 confirms the written request requirement for all Brussels leases.

Wallonia

The Decree of 15 March 2018 applies the same rule. A registered letter is recommended but a regular email with confirmation suffices.

Flanders

The Flemish Housing Rental Decree of 9 November 2018 confirms that indexation requires a written request. For leases in Flanders, the request should be in Dutch.