When can a rent be indexed in Belgium?

Answer in brief

In Belgium, a landlord can index the rent once a year, on the anniversary date of the effective start of the lease (not the signature), and only on a written request to the tenant. The new rent applies at the earliest on that anniversary date, once requested in writing — never before.

This article answers the question when can a rent be indexed from all angles: anniversary date, frequency, mandatory written document, time windows according to the lease type.

The anniversary date: signature or effective start?

This is the most frequent confusion, and it has direct consequences on the possible indexation date.

The legal rule is clear: the date to remember is that of the effective start of the lease (often called “taking effect” or “occupation date”), not that of the signature. Article 1728^bis of the Belgian Civil Code and the Law of 6 August 1993 are unambiguous on this point.

Concrete example

Let’s imagine a Walloon residential lease:

• Signature date: 14 June 2021
• Effective start date (handover of keys, beginning of occupation): 1 July 2021

The first possible indexation is not on 14 June 2022, but indeed on 1 July 2022. All subsequent indexations will take place each 1 July (2023, 2024, etc.), not each 14 June.

Signature date	Effective start date	Annual indexation date
14 June 2021	1 July 2021	1 July (2022, 2023, 2024…)
28 February 2022	1 March 2022	1 March (each year)
5 September 2023	5 September 2023 (immediate)	5 September (each year)
12 January 2024	1 February 2024	1 February (each year)

Failing an effective start date specified in the lease, the signature date is taken by default. That is why it is crucial that the lease explicitly mentions both dates.

Our step-by-step calculation guide details how to find the right health indexes based on this anniversary date.

Once a year only, no accumulation

The indexation frequency is strictly regulated: one indexation per rental year, on each anniversary of the effective start. A late request does not shift the next anniversary. It is not possible to anticipate, nor to “double” the following year in case of oversight.

Concretely:

• No accumulation: if you forget to index a year, you cannot apply two indexations the following year
• No anticipation: impossible to index before the anniversary date, even if the health index has greatly increased
• No “fractioning”: no partial indexation in mid-year

In case of oversight, the retroactive effect of the request is limited to the 3 months preceding the month of the request (art. 1728bis, §1 of the Belgian Civil Code). Our rent indexation after 5 years guide explains this retroactivity rule in detail: regardless of how many years have been forgotten, only the last 3 months are recoverable.

The mandatory written document

Indexation is not automatic: the landlord must request it in writing to the tenant. Without a written request, the rent stays at its initial amount — and the tenant has the right to refuse a non-notified retroactive regularisation.

Free form, registered mail recommended

The form of the letter is legally free:

• A simple letter technically suffices (art. 1728^bis does not specify a particular form)
• An email can do if the tenant expressly accepts it or responds to it
• Registered mail with acknowledgment of receipt remains strongly recommended for proof

Why favour registered mail? Because the month of the written request fixes the 3-month retroactivity window (art. 1728^bis, §1). Without registered mail, in case of dispute, you have no formal proof of the date of your request.

Our notification letter template is compliant with legal requirements and includes all the elements to mention (legal references, calculation, effective date).

Minimum content of the letter

• Reference to art. 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 the signature and month preceding the indexation)
• Calculation detail and new monthly rent
• Effective date of the new rent (the anniversary date)

Time windows according to the lease type

The question when can a rent be indexed also depends on the lease type. Here are the main configurations in Belgian law.

9-year lease (standard residential lease)

This is the standard lease for main residence rentals in Belgium. Its structure allows 9 possible indexations, one on each anniversary date of the effective start.

Example: lease signed in June 2021, effective start 1 July 2021. Possible indexations on 1 July 2022, 2023, 2024, 2025, 2026, 2027, 2028, 2029 and 2030 (last year of the lease).

Short-duration lease (≤ 3 years)

The short-duration main residence lease (3 years or less, non-renewable beyond 3 cumulative years) allows:

• 0 indexation for a 1-year lease (the first anniversary coincides with the end of the lease; an extension reopens the right)
• 1 indexation for a 2-year lease
• 2 indexations for a 3-year lease

On each anniversary date of the effective start, as for the 9-year lease.

Lifetime lease

Rare but existing, the lifetime lease allows annual indexations as long as the lease is in force. No time cap — each anniversary date opens the right to an indexation, until the end of the lease (generally by the tenant’s death).

Rolling or tacitly renewed lease

If a lease is tacitly renewed or “rolls” (for example a short-duration lease that extends without a new contract), the anniversary date remains that of the original lease. The counter does not restart from zero: it is the continuity of the same lease.

Concretely: a lease signed on 1 March 2020 and tacitly renewed in 2023 keeps its annual indexation date on 1 March.

Special case: indefinite-term lease

Rarer, but the indefinite-term lease exists in Belgium (main residence rentals prior to 1991, requalified verbal agreements, etc.). Indexation remains possible under the usual conditions:

• In Brussels and Wallonia, a registered lease (without registration, no indexation — see our unregistered lease guide)
• Indexation clause present or not excluded
• Anniversary date taken = signature date (failing defined effective start)
• Once a year, in writing

The annual rhythm remains the absolute rule, even in this indefinite framework.

Summary table of cases

Lease type	First possible indexation	Frequency
9-year lease (residential)	1 year after effective start	Annual, 8 times (the 9th anniversary is the end of the lease)
Short-duration 1-year	—	None (unless extended)
Short-duration 2-year	1 year after effective start	1 time
Short-duration 3-year	1 year after effective start	Annual, 2 times
Lifetime lease	1 year after effective start	Annual, no cap
Indefinite-term lease	1 year after signature (default)	Annual, no cap

All these cases assume that the indexation clause is not excluded — and, in Brussels and Wallonia, that the lease is registered.

To immediately calculate your new indexation from your anniversary date, use our indexation calculator. It automatically applies the official formula (art. 1728^bis Civil Code) with up-to-date Statbel health indexes, in accordance with the Royal Decree of 24 December 1993.

Going further

• How to calculate an indexation step-by-step
• Indexation after 5 years: rules and catch-up
• Unregistered lease: can it be indexed?
• Notification letter template
• Complete calculation methodology
• Our indexation calculator — calculation from your data

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