In Belgium

The concept of retroactive effect primarily arises in the context of rent indexation. The rule is simple: if the landlord forgets to request indexation at the lease anniversary, they can only claim it retroactively for the 3 months preceding their written request.

In practice, a landlord who has not indexed for 2 years can only claim the last 3 months of indexation — the earlier months are lost. This protects the tenant against massive and late claims.

Notice periods can never have retroactive effect: they take effect on the 1st of the month following receipt, never before.

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Warning
The 3-month limitation is a ceiling, not an automatic right. The landlord must always send a written request to the tenant. Without a request, indexation does not apply, even retroactively.

Practical example

A landlord forgets to index the rent for 18 months. The indexation would have increased rent by 25 EUR/month. They send their request on 1 March. They can only claim 3 x 25 EUR = 75 EUR (December, January, February). The 15 earlier months are lost — a 375 EUR shortfall. Indexation applies normally from April onwards.