Can a rent still be indexed after 5 years without indexation?

Answer in brief

Indexation is not automatic in Belgium: it is a right that the landlord must actively exercise in writing. If you have not requested it for several years, you can still claim a catch-up — but the arrears are limited to the three months preceding the month of the request (art. 1728bis, §1 of the Belgian Civil Code). For the future, however, the request applies in full: the rent moves to the fully indexed amount.

Indexation is not automatic

Many landlords think that indexation is automatically applied each year on the lease’s anniversary date. That is wrong. Article 1728^bis of the Belgian Civil Code requires an active step from the landlord:

• Indexation must be requested in writing to the tenant
• It takes effect from the receipt of the request, not before
• Without a written request, the rent remains at its initial amount — and the tenant has the right to refuse a non-notified retroactive indexation

It is this obligation of active request that creates the possibility — or impossibility — of catching up.

Retroactivity limited to three months

Article 1728bis, §1 of the Belgian Civil Code is explicit: the indexation request only has retroactive effect for the three months preceding the month of the request. All arrears older than these three months are permanently lost.

Here is how it applies concretely:

Lease signed	Expected annual indexations	In 2026 you can claim
June 2018	8 indexations (2019-2026)	The arrears of the 3 months preceding the request
June 2021	5 indexations (2022-2026)	The arrears of the 3 months preceding the request
June 2024	2 indexations (2025-2026)	The arrears of the 3 months preceding the request

Result: regardless of how many years you “forgot”, you only recover three months of arrears. A late catch-up request has no retroactive effect beyond — but it sets the future rent at the fully indexed amount.

Why this strict limit?

The aim of the law is twofold:

1. Tenant protection: avoid a landlord brutally claiming several years of cumulated arrears, which could represent considerable sums
2. Legal certainty: a rent not indexed for years becomes the de facto rent — the tenant has organised their budget accordingly

Catch-up calculation: concrete example

Take the case of a Walloon lease signed on 1 June 2018 for the amount of €750, never indexed for 8 years. In June 2026, the landlord discovers the oversight and wants to catch up.

Step 1 — Identify the indexes:

• Base index = May 2018 (month preceding the signature): 106.99
• New index = May 2026 (month preceding the 2026 anniversary date): 132.47

Step 2 — Apply the formula:

• Theoretical indexed rent = 750 × (132.47 ÷ 106.99) = €928.68
• Monthly difference = 928.68 − 750 = €178.68/month

Step 3 — Calculate the recoverable arrears (3 months preceding the month of the request):

• Period March 2026-May 2026: 3 × 178.68 = €536.04

Step 4 — From notification:

• From June 2026, the monthly rent rises to €928.68 (to be paid each following month)

To do this calculation automatically with your lease:

How to notify the catch-up to the tenant

The form of the letter is free, but we systematically recommend registered mail with acknowledgment of receipt for two reasons:

1. Date proof: the three-month retroactivity is calculated from the receipt by the tenant, not from the dispatch. Registered mail prevails
2. Anticipation of dispute: if the tenant refuses, you will need the proof before the justice of the peace

The content must mandatorily mention:

• Reference to article 1728^bis of the Belgian Civil Code and to the Law of 6 August 1993
• Signature and effective start date of the lease
• Health indexes used (month preceding signature + month preceding catch-up)
• Calculation detail + new monthly rent
• Effective date of the new rent

A notification letter template is available, as well as a registered mail slip to attach.

Special case: unregistered lease

If your lease was never registered with the competent registration office (legal obligation within 2 months of the signature), the tenant has the right to refuse even the catch-up of the last three months.

Registration is an absolute prerequisite for the exercise of the indexation right. Without it, indexation is not due — regardless of the elapsed duration or the formula used.

Our unregistered lease guide details the possible remedies in this case (retroactive registration, tax consequences for the landlord, etc.).

If the tenant refuses the catch-up

Three common scenarios:

1. The tenant refuses on a wrong ground (e.g. “you should have warned me earlier”, or “5 years is too late”): your catch-up right over the last 3 months is well-founded. Procedure: registered formal notice → justice of the peace if no payment within 15 days

2. The tenant refuses on a valid ground (unregistered lease, miscalculation): they are right on the merits. First regularise the ground (late registration possible, correct recalculation), then re-notify

3. The tenant ignores the letter: send a formal notice letter by registered mail with a 15-day deadline. Without response, refer to the justice of the peace of the property’s commune (free procedure, hearing in 4-8 weeks)

Our tenant refusal guide covers these 3 cases in detail with the associated letter templates.

Going further

• How to calculate an indexation step-by-step
• When can a rent be indexed?
• Notification letter template
• Complete calculation methodology