When to notify the tenant of an indexation?
Belgium: no mandatory legal notice. But the optimal notification timing conditions legal security and the quality of the rental relationship.
Brief answer
In Belgium, no legal notice is mandatory to notify an indexation to the tenant (article 1728bis of the Civil Code). Notification is valid before, on the day itself, or after the anniversary date. The recommended optimal timing is 1 to 2 months before the anniversary date: this window reconciles legal security, calculation precision (the health index must be published) and quality of relationship with the tenant.
Practical golden rule: send the registered mail between 30 and 60 days before the anniversary date. Too early = index unavailable. Too late = unnecessary monthly losses.
The false myth of « mandatory prior notice »
This is the most widespread received idea among new landlords: a minimum legal deadline would have to be respected. It is false.
Article 1728bis of the Belgian Civil Code mentions no prior notice:
« § 1er. (…) the rent is adapted to the cost of living once per rental year, on the anniversary date of entry into force of the lease. (…) Indexation takes effect from the month following the request. »
The notification produces its effects the month following its receipt, period. No anticipation is required. The confusion probably comes from:
- Other contractual formalities (termination notice = 3 months on the tenant’s side, for example)
- Recommended commercial practice (1-2 months in advance) wrongly elevated to a rule of law
- Foreign systems (in France notably, certain leases impose a specific contractual mention)
In Belgian law: zero legal prior notice.
Optimal timing in practice: 1 to 2 months before
Although no deadline is legally required, intelligent timing maximises both legal security and quality of the rental relationship. Here is the ideal window:
| Moment | Advantages | Disadvantages |
|---|---|---|
| 3+ months before anniversary | Maximum anticipation | Exact health index not yet published |
| 2 months before | Precise calculation + digestion time | None |
| 1 month before | Ideal for standing order adjustment | None |
| 2 weeks before | Late but valid | Tenant may be surprised |
| The day itself | Legally valid | No room for discussion |
| 1 month after | Recoverable (3-month retroactive effect) | Recovery must be explicitly requested |
| 6 months after | Loss of 3 months | Retroactivity limited to 3 months (art. 1728bis, §1) |
The 30-60 days before anniversary window combines all advantages: the health index of the month preceding the anniversary is generally published (Statbel releases around the 26th of each month), the tenant has time to adjust their standing order, and the relationship remains fluid.
Why not notify 6 months in advance?
The health index published by Statbel is not predictive data: it is calculated a posteriori on the actual prices of the past month, and published around the 26th of the following month.
Consequence: to index on 1 June 2026, the May 2026 index is needed, which is only published around 26 May 2026. Notifying in March 2026 would force using an estimate — and therefore risk an incorrect calculation.
| You want to index on | Index needed | Available at Statbel |
|---|---|---|
| 1 January 2026 | December 2025 | ~26 December 2025 |
| 1 June 2026 | May 2026 | ~26 May 2026 |
| 1 September 2026 | August 2026 | ~26 August 2026 |
This is why the 30-60 days before window is optimal: it is compatible with the index publication cadence, without being too late.
The 3-step method
Step 1: Automatic reminder 60 days before
Schedule a recurring reminder in your calendar: « indexation [property address] »
each year on anniversary date − 60 days. This is the most profitable organisational
investment in the landlord’s job.
Step 2: Verification of the index of the month preceding the anniversary
As soon as Statbel publishes (around the 26th), note the published health index and use the calculator to confirm the new rent.
Step 3: Send the registered mail between D-30 and D-45
Mail the notification letter (ideally registered with acknowledgement of receipt) between 30 and 45 days before the anniversary date. Postal delivery time: 2-4 working days in Belgium.
Our notification template includes all mandatory elements (detailed calculation, legal references, IBAN).
Bonus: the « informal notice » to preserve the relationship
For landlords who prioritise the quality of the rental relationship (long-term tenants, good payers), a prior informal notification is very effective:
-
6 weeks before: send a friendly email to the tenant
« Hello, I wanted to inform you in advance that on your next lease anniversary date (the X), I will proceed with the annual rent indexation. The indicative estimate is X €/month (variation of about X € compared to today). I will send you the official detailed calculation by registered mail within 4 weeks. Don’t hesitate if you have questions. »
-
3 weeks before: send the official registered mail with exact calculation
This approach halves contestations in practice. It costs 5 minutes and does not affect the legal validity of the subsequent notification at all.
Special cases
Notification after the anniversary date
Possible and legally valid, but with loss of indexed rent for the period prior to the request’s 3-month retroactive effect (art. 1728bis, §1). Example:
- Lease signed 1 June 2017
- Indexation should have been notified around May 2026 for the 1 June anniversary
- Effective notification on 15 October 2026
- → the request reaches back over the 3 months preceding the month of the request: July, August and September 2026 remain recoverable
- → net loss of 1 month of indexed rent (June 2026)
For an indexed rent increasing by 25 €/month: loss = 25 € through procrastination alone — and it grows by one more month for every additional month of waiting.
Very late notification (prolonged omission)
Beyond 3 months after the anniversary date, see our retroactive indexation guide — article 1728bis, §1 of the Civil Code caps recovery at the 3 months preceding the month of the request.
Anticipated notification (more than 3 months before)
Legally possible but erroneous calculation guaranteed: the health index of the month preceding the anniversary is not yet published. If you index on the basis of an older index, you underestimate the new rent. The tenant can then refuse to overpay or demand an official recalculation.
Rental vacancy between two tenants
Indexation attaches to the lease, not to the tenant. If the property remains empty for a few months between two leases, the lease ends and a new lease starts with a new negotiated initial rent — no indexation to notify the new tenant before their first anniversary date.
Annual calendar: the methodical landlord’s year
Here is the calendar used by professional real estate agencies, to adapt according to the anniversary date of each lease:
| Month (relative to anniversary) | Action |
|---|---|
| Anniv − 90 days | Calendar reminder |
| Anniv − 60 days | Verification of last notification + forecast calculation |
| Anniv − 45 days | Informal email to tenant (optional) |
| Anniv − 30 days | Verification of index publication (Statbel) |
| Anniv − 25 days | Send registered mail with official calculation |
| Anniv | Effective date (or month following receipt) |
| Anniv + 7 days | Check that the tenant has adjusted the standing order |
For 5 or 10 properties, this calendar becomes indispensable. For 1 property, a simple D-60 reminder in Google Calendar suffices.
To go further
- When can one index a rent? — frequency and date rules
- How to notify the tenant — mandatory elements
- Retroactive indexation: rules — if prolonged delay
- Notification letter template — ready-to-sign template
- Calculate the new rent — uses up-to-date Statbel indexes
- Tenant refusal: remedies — if disagreement
- The law on indexation — detailed legal sources
- Methodology — reference calculation
Perfect timing does not exist in law, but exists in practice: the 30-60 days before anniversary window is the best compromise between technical precision and relational comfort.
Frequently asked questions
Does Belgian law impose a prior notice before the anniversary date to notify indexation? + −
No. Article 1728bis of the Civil Code imposes no prior deadline. The notification can occur before, on the day itself or after the anniversary date, with no effect on legal validity. This is one of the notable particularities of Belgian law — many landlords wrongly believe a prior notice is mandatory (probable confusion with other contractual formalities).
What is the recommended deadline in practice? + −
1 to 2 months before the anniversary date. This window allows the tenant to absorb the shock, anticipate the standing order adjustment, and possibly ask questions. Beyond 3 months in advance, the exact health index is not yet known (monthly Statbel publication ~26th of the month) — the notification would be imprecise. Before 1 month or after the anniversary, it is legally valid but commercially less readable.
What happens if I notify after the anniversary date? + −
Indexation remains valid: the request has retroactive effect limited to the 3 months preceding the month of the request (article 1728bis, §1 of the Civil Code). If you notify 2 months after the anniversary, those months therefore remain recoverable. Beyond 3 months of delay, however, the exceeding months are permanently lost. For details, see our retroactive indexation guide.
Should one send an informal notice before the official registered mail? + −
Not mandatory but excellent for the rental relationship. A friendly email 4 to 6 weeks before the official notification, explaining that indexation is coming and giving an estimate of the new rent, defuses tensions. Followed by the official registered mail 2 weeks later, with detailed calculation. Landlords who practice this two-step approach report fewer contestations and better rental stability.
If the tenant is in financial difficulty, can the notification be delayed? + −
Yes, the landlord has the right to waive or defer an indexation, totally or partially, with no legal precedent for the following years. You can decide to wait 6 months or even a year if the tenant's situation justifies it. But beware: beyond 3 months without notification, retroactivity caps — the request only reaches back to the 3 months preceding it (art. 1728bis, §1), and you will no longer recover the entire accumulated delay.
And for a new tenant arrived during the lease (assignment, authorised sublease)? + −
The anniversary date remains that of the original signature of the lease, not the new tenant's arrival date. You notify on the same date as before, sending the letter to the new tenant. For a first notification after assignment, slipping into the letter an informative mention on the history of past indexations reassures and avoids questions.
Calculate your indexation
Our tool automatically applies the Belgian legal formula with up-to-date Statbel indexes.
Go to the calculator →This article is informational and does not constitute legal advice. For any disputed or complex situation, consult a lawyer or the justice of the peace of the property's commune (free procedure).