How to notify the indexation to the tenant

Answer in brief

In Belgium, the indexation notification to the tenant must be made in writing (Law of 6 August 1993, article 1728^bis of the Civil Code). The form is free — simple letter or registered — but the letter must contain 5 mandatory mentions without which it can be disputed. Indexation only takes effect from receipt by the tenant, never retroactively.

The mandatory mentions of the letter

For an indexation notification to be legally valid, the letter must contain these five elements — any failing can justify a dispute by the tenant:

• Full identity of the landlord: name, first name (or company name) and postal address of the owner or their representative (agency, syndic, notary)
• Tenant’s identity: name, first name and notification address (in principle, the rented property’s address, unless the tenant has provided another address for their correspondence)
• Precise lease reference: signature date, effective start date, and exact address of the rented property — these three elements allow unambiguously identifying the contract concerned by the indexation
• Legal reference: mention of article 1728^bis of the Belgian Civil Code and of the Law of 6 August 1993 on lease contracts (this mention is theoretically optional, but it prevents bad-faith disputes)
• Complete calculation detail: initial rent, base health index + reference month, new health index + reference month, applied formula, EPC coefficient if applicable (Wallonia and Brussels), and resulting new monthly rent

To these five mentions are added two strongly recommended but not strictly mandatory elements: the effective date of the new rent, and the landlord’s handwritten signature at the bottom of the letter.

Registered or simple letter?

Belgian law does not impose registered mail. Yet, almost all lawyers specialised in leases systematically recommend it. Here is the concrete comparison:

Criterion	Simple letter	Registered with AR
Cost (2026)	~€1.80	€7.50 - 10
Legal validity	Yes	Yes
Proof of dispatch	No	Yes
Proof of receipt	No	Yes (acknowledgment of receipt)
Date opposable to tenant	Disputable	Receipt date prevails
Recommended in case of dispute	No	Yes (justice of the peace)

Our recommendation: send by registered mail with acknowledgment of receipt for first exchanges with a tenant and systematically in case of foreseeable conflict (tenant in payment arrears, tense relationship, catch-up over several years). For long-term tenants with whom the relationship is excellent, a simple letter may suffice — but always keep a dated and signed copy for your archives.

For the detail of the registered mail formalism (dispatch slip, withdrawal at La Poste), see our registered mail guide.

When to send the letter

The ideal is to send the indexation notification 1 to 2 months before the lease’s anniversary date. This anticipation allows the tenant to absorb the change, possibly ask their questions, and the landlord to manage a refusal or request for explanation before the actual deadline.

At the latest, the letter must be sent on the anniversary date itself. Any delay loses monthly payments, because indexation only takes effect from the receipt of the notification — not from the anniversary date itself. That is why we detail the exact window in our when to index a rent guide.

If you discover a delay of several months or several years, know that the retroactive effect of the request is strictly limited to the 3 months preceding the month of the request (article 1728bis, §1 of the Civil Code). Catching up several years is not possible — see our rent indexation after 5 years guide for the detail.

Sample letter commented section by section

Here is a sample letter, presented in blocks, with the role of each section:

[Landlord header] Jean Dupont Rue de la Loi 100 1000 Brussels

[Place and date] Brussels, 15 May 2026

[Recipient] Madam Marie Martin Avenue de la Toison d’Or 25 1050 Ixelles

[Dispatch mode] Registered letter with acknowledgment of receipt

[Subject] Annual rent indexation — lease of 1 June 2024

[Letter body] Dear Madam,

In accordance with article 1728^bis of the Belgian Civil Code and the Law of 6 August 1993 on lease contracts, I notify you of the annual indexation of the rent for the property located at Avenue de la Toison d’Or 25, 1050 Ixelles, of which we signed the lease on 1 June 2024 with effective start on the same date.

The calculation is as follows:

• Initial rent: €950.00
• Health index of May 2024 (month preceding the signature): 128.12
• Health index of May 2026 (month preceding the anniversary): 132.47
• Formula: 950 × (132.47 ÷ 128.12) = €982.26

The new monthly rent is therefore €982.26, from 1 June 2026.

Please adjust your next transfer accordingly and I remain available for any question.

Yours faithfully,

[Handwritten signature] Jean Dupont

Header block: name and full address of the landlord. If you go through an agency or syndic, mention their details as a complement.

Recipient block: tenant’s notification address. By default, it is the rented property’s address — see the section below on address changes.

Subject block: unambiguous reference to the concerned lease. Avoid vague formulations like “your rent” — specify the signature date.

Body block: contains the five mandatory mentions. The calculation detail is essential; without it, the rent indexation letter can be disputed for insufficient information.

Signature block: handwritten if possible, especially for postal mail. A qualified electronic signature (eIDAS) remains the legal equivalent for digital sendings.

A ready-to-sign letter template in PDF format is available, pre-filled with the values of your calculation if you generated it via the calculator.

And if the tenant doesn’t respond?

Important point often misunderstood: the tenant’s agreement is not required. Indexation is a landlord’s right provided by the lease and the law. Once notification is received, the new rent applies as of right from the next monthly payment — regardless of whether the tenant signs an acknowledgment of receipt, responds positively or ignores the letter.

Three scenarios if the tenant does not pay the new amount:

1. The tenant continues to pay the old rent without explicitly disputing: send a registered formal notice reminding the calculation and demanding the catch-up of the differential
2. The tenant explicitly disputes the calculation: verify that all elements are correct (indexes, formula, dates), respond in writing with the Statbel evidence
3. The tenant ignores everything: after a formal notice without effect, refer to the justice of the peace of the property’s commune (free procedure, hearing within 4 to 8 weeks)

Our tenant refusal guide covers these scenarios in detail with the reminder and formal notice letter templates.

Special case: the tenant has changed address

If the tenant is still in the rented property but has provided another postal address (for example a family member’s, or a professional post box):

• Notifying at the rented property’s address is legally valid — the property’s address is always considered the default notification address
• Notifying at the address provided by the tenant is even more solid — the tenant will not be able to dispute either receipt or knowledge of the content

If the tenant has left the property (prohibited subletting, unnotified move), the notification remains valid at the rented property’s address until proof to the contrary. The indexation letter therefore retains its validity, and the tenant remains liable for the new rent.

Going further

• Notification letter template (PDF)
• How to calculate an indexation step-by-step
• When can a rent be indexed?
• What to do if the tenant refuses the indexation
• Indexation after several years of oversight
• Registered mail dispatch slip

To generate a notification letter pre-filled with your calculation, use our tool: