Must the landlord notify the tenant before indexing?

Written notice is mandatory

Key rules regarding the indexation notice:

• The request must be in writing (letter, email, registered mail)
• It must be sent on or after the anniversary date
• It must contain the detailed calculation (base rent, indices, result)
• The indexed rent applies from the date of the request, not retroactively (beyond 3 months)
• Indexation is never automatic, even with a lease clause

What the notice must include

A proper indexation notice should contain:

1. Reference to the lease: date, property address
2. Base rent: the amount from the original lease
3. Starting health index: value and reference month
4. New health index: value and reference month
5. Calculation: Base rent x (New index / Starting index) = New rent
6. Effective date: the date from which the new rent applies
7. Source reference: link to Statbel for index verification

Consequences of failing to notify

If the landlord applies indexation without sending a written notice:

• The tenant can refuse to pay the indexed portion
• The tenant is not in default for paying only the non-indexed rent
• The landlord cannot claim arrears for the period before the written request
• The indexation only takes effect from the date of the written request
• The retroactive effect is limited to 3 months before the request in Brussels and Wallonia

This protection exists to prevent tenants from facing unexpected rent increases without proper notice and the ability to verify the calculation.

Regional specifics

Brussels-Capital Region

The Ordinance of 27 July 2017 reinforces the written request requirement. The notice must be sent to the tenant’s address as stated in the lease.

Wallonia

The Decree of 15 March 2018 applies the same rules. A registered letter is recommended but not mandatory for the notice.

Flanders

The Flemish Housing Rental Decree of 9 November 2018 applies the written request rule. The notice should be in Dutch for leases in Flanders.