Can you catch up on forgotten indexation from previous years?

Q: Can you catch up on forgotten indexation from previous years?

A landlord who forgot to index can always restart indexation going forward by applying the legal formula from the original base rent. However, retroactivity is limited: in Brussels and Wallonia, indexation only takes effect for the 3 months preceding the written request. In Flanders, the 5-year civil prescription applies. Amounts not claimed beyond these deadlines are permanently lost.

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

Quick answer

Partially. The landlord can restart indexation going forward, but retroactivity is limited to 3 months in Brussels and Wallonia. Unclaimed years are permanently lost.

Can you catch up on forgotten indexation from previous years?

Illustration — Can you catch up on forgotten indexation from previous years? alt="can you catch up on forgotten indexation from previous years?"

Partial catch-up is possible

Quick answer

You can restart indexation going forward at any time using the original base rent and current health index. However, retroactivity is limited to 3 months in Brussels and Wallonia. The lost months between anniversaries and your written request are gone permanently.

The key distinction is between going-forward and retroactive indexation:

Going forward: always possible, using the original base rent and current index
Retroactive (Brussels/Wallonia): limited to 3 months before the written request
Retroactive (Flanders): 5-year prescription, but practical effect still limited

How to restart indexation after a gap

Find the base rent from the original lease
Identify the current health index on Statbel (month preceding the latest anniversary)
Apply the formula: Base rent x (Current index / Starting index) = New indexed rent
Send a written request to the tenant with the full calculation
The new rent takes effect from the date of the request (retroactive to 3 months maximum)

The good news: the going-forward calculation reflects all cumulative inflation since the lease started, so the new indexed rent will be higher than if you had been indexing annually all along.

Practical tip

Set up automatic indexation reminders to avoid future missed indexations. Even one forgotten year can cost hundreds of euros in permanently lost rental income.

Financial impact of delayed catch-up

For a base rent of 800 EUR, if you forgot to index for 3 years and then send a written request:

New indexed rent (going forward): correct amount based on cumulative inflation
Retroactive recovery: only 3 months of the difference between old and new rent
Permanently lost: all the monthly differences for the years you did not claim

The longer you wait, the more income is permanently lost. Act as soon as you realise the omission.

Regional specifics

Brussels-Capital Region

Under the Ordinance of 27 July 2017, the 3-month retroactive limit is strictly applied.

Wallonia

The Decree of 15 March 2018 applies the same 3-month rule.

Flanders

The Flemish Housing Rental Decree of 9 November 2018 applies a 5-year civil prescription, but the retroactive effect is generally also limited in practice.

Legal basis

Article 1728bis of the Belgian Civil Code and regional implementations. The retroactive limitation protects tenants from sudden large back-payment demands.