Indexing a rent is not raising it at will: it is readjusting it according to a legal formula tied to inflation. The mechanism protects the tenant (rent does not jump) as much as the landlord (rent does not lose value over time).
The health index, not raw inflation
Indexation does not follow just any index, but the health index — the consumer price index from which certain products (tobacco, alcohol, fuels) have been removed. It is the same index used to index wages. It is published each month by Statbel.
The formula
The calculation is set by law:
New rent = base rent × health index of the month preceding the anniversary of the lease / health index of the month preceding the lease's entry into force
The "base rent" is the one agreed at the outset, and the "starting index" is frozen at signing. So you never re-index on last year's already indexed rent: you always start again from the base rent and the initial index. The FPS Economy provides an official calculator that applies this formula.
The conditions
Indexation is only possible if:
- the lease is written (a verbal lease is not indexed);
- the periodicity is respected: once a year at most, at the earliest on the anniversary date of the lease's entry into force;
- the landlord requests it: it is not automatic. Failing a request, it can only be recovered over a limited period backwards, not over all the forgotten years.
A tenant who receives an indexation request can therefore check the calculation: an incorrectly calculated indexation is not owed.
Energy performance changes things
In recent years, several Regions have made conditional or capped the indexation of homes with poor energy labels: depending on the Region and the period, a poorly rated home could not be indexed, or only partially. These measures are evolving. Before indexing, check the regime in force in the home's Region and the property's label.
What indexation does not do
Indexation follows inflation, not the market. If rents in the municipality have risen sharply, indexation does not make up that gap: it does not allow a rent undervalued against the market to be realigned. That realignment comes under other, more tightly framed mechanisms — and first requires knowing where the property sits within the prices practised locally.