Retroactive indexation is limited
In Brussels and Wallonia, indexation only takes effect for the 3 months preceding the written request. In Flanders, the 5-year prescription applies but retroactivity remains limited. Unclaimed years are permanently lost — you cannot recover them.
The rules on retroactive indexation vary by region:
| Region | Retroactive period | Prescription |
|---|---|---|
| Brussels | 3 months | 3 months |
| Wallonia | 3 months | 3 months |
| Flanders | 3 months (effect) | 5 years (right) |
Even if you have the right to index for a longer period in Flanders, the practical effect is still limited. The lesson is clear: always index on time.
How to claim retroactive indexation
If you have forgotten to index for one or more years:
- Calculate the current indexed rent using the indexation formula (always from the base rent, never from a previously indexed amount)
- Send a written request to the tenant with the detailed calculation
- Specify the effective date: the indexed rent applies from the date of the request (or 3 months before at most)
- Apply the new indexed rent from the next payment due after the request
The written request must include the base rent, the starting health index, the new health index and the calculated result.
Even if you lost years of indexation, the going-forward calculation always uses the original base rent and the current health index. The result will reflect all cumulated inflation since the lease began, even if past indexations were not claimed.
Financial impact of forgotten indexation
The financial loss from forgotten indexation can be significant. For a base rent of 800 EUR with annual inflation of approximately 2.5%:
| Years forgotten | Lost monthly amount | Lost annually |
|---|---|---|
| 1 year | ~20 EUR/month | ~240 EUR |
| 3 years | ~60 EUR/month | ~720 EUR |
| 5 years | ~100 EUR/month | ~1,200 EUR |
These amounts are permanently lost. Setting up automatic indexation reminders through property management software prevents this loss.
Regional specifics
Brussels-Capital Region
Under the Ordinance of 27 July 2017, the retroactive effect is strictly limited to 3 months. Any indexation beyond this period is lost.
Wallonia
The Decree of 15 March 2018 applies the same 3-month retroactive limit as Brussels.
Flanders
The Flemish Housing Rental Decree of 9 November 2018 applies a 5-year civil prescription, but the retroactive effect of the indexation request remains generally limited to 3 months in practice.
Article 1728bis of the Belgian Civil Code and regional implementations. The 3-month retroactive limit is a key protection for tenants against sudden large back-payments.