Retroactive rent indexation: rules and limits
What rules govern a retroactive indexation in Belgium? 3-month cap, recovery procedure, worked examples and risks.
Brief answer
In Belgium, an indexation request only has retroactive effect for the 3 months preceding the month of the request (article 1728bis, §1 of the Civil Code, a rule confirmed by all three regions). Whatever the duration of omission (2, 5 or 10 years), only those 3 months of arrears can be claimed from the tenant — the rest is definitively lost. On the other hand, the future rent is fully preserved: from the request onwards, it is calculated with the full formula from the base rent.
Immediate reflex: if you discover an omission, send a written request right now (registered mail advisable). Each additional month of delay shifts the 3-month window and definitively costs you one more month of arrears.
The principle: 3 months, no more
The historical basis is article 1728bis, §1 of the Belgian Civil Code: the indexation request only has retroactive effect for the three months preceding the month of the request. All three regions apply the same limit:
- Wallonia (wallonie.be): a landlord who forgets to index at the lease anniversary can still do so, but the right to indexation arrears is limited to the last three months preceding the month of the request.
- Brussels (be.brussels): retroactivity limited to the three months preceding the month of the request.
- Flanders: article 34 of the Vlaams Woninghuurdecreet, the same 3-month rule.
Amounts that remain due after a regular request are, for their part, subject to prescription under the general rules of the Civil Code.
Three concrete scenarios:
| Duration of omission | Indexations theoretically due | Legally recoverable |
|---|---|---|
| 2 years | 2 annual indexations | The 3 months preceding the month of the request |
| 5 years | 5 annual indexations | The 3 months preceding the month of the request |
| 10 years | 10 annual indexations | The 3 months preceding the month of the request |
This is the most radical effect of the rule: a landlord who « forgets » three years only recovers three months of arrears. But for the future, the request takes full effect: the future rent is recalculated with the full formula from the base rent — the omission never reduces the future rent.
When does the 3-month retroactivity start?
From the date of the written request sent to the tenant. Not before. Concretely:
- You send your request on 15 May 2026 (registered mail advisable)
- Retroactivity covers the three months preceding the month of the request: February, March and April 2026
- All arrears prior to that window are lost
The lease signature date, the anniversary date, or the date of each forgotten indexation do not count. Only the date of the written request sets the window.
This is why sending by registered mail with acknowledgement of receipt remains strongly advisable: in case of dispute, the AR provides formal proof of the date of the request and of its receipt. With a simple letter, the tenant can contest the date — and the retroactivity window then plays in their favour.
Calculating a 3-month recovery: worked example
Typical case: lease signed in June 2018, never indexed. Initial rent 800 €. Request sent in June 2026.
Step 1: rebuild the chain of theoretical annual indexations
| Year | Health index June | Theoretical rent (cascading) |
|---|---|---|
| 2018 (signature) | 106.87 (base 2013) | 800.00 € |
| 2019 | 109.21 | 817.52 € |
| 2020 | 110.54 | 827.48 € |
| 2021 | 113.29 | 847.96 € |
| 2022 | 121.68 | 910.81 € |
| 2023 | 128.40 | 961.11 € |
| 2024 | 131.55 | 984.69 € |
| 2025 | 134.54 | 1,007.15 € |
| 2026 | 137.30 | 1,027.80 € |
The theoretical 2026 rent is 1,027.80 € after 8 cascading indexations. Without recovery, the tenant continues to pay 800 €. Monthly difference: +227.80 €/month.
This cascading chain is pedagogical: mathematically, the result is identical to the direct formula base rent × new index / starting index (800 € × 137.30 / 106.87 = 1,027.80 €). That formula is what determines the future rent.
Step 2: apply the 3-month cap
Over 8 years of omission, the landlord can only claim the 3 months preceding the month of the request (March, April and May 2026):
- Theoretical rent applicable over those months: ~1,007 € (last matured indexation, 2025)
- Rent actually paid: 800 €
- Difference: ~207 €/month × 3 months = 621 € of recoverable arrears
For the whole rest of the period (June 2018 → February 2026), the arrears are definitively lost.
Step 3: ask for the new rent for the future
For the future, the new effective rent is 1,027.80 €/month from June 2026, calculated with the full formula from the base rent. The historical rent loss is not recovered, but the future is fully preserved: acting fast limits the loss, and the future rent follows the full theoretical indexation.
To calculate your exact case, use our calculator: enter the original signature date and the current indexation date — the result applies the full formula.
Retroactive request procedure
1. Rebuild the indexation chain
Use the calculator for each past anniversary year. Keep the screenshots index by index, which will serve as proof if the tenant contests the calculation.
2. Draft the request letter
The request must contain:
- The original signature date
- The base index (month preceding signature)
- The new index (month preceding the date of the request)
- The detailed calculation of the indexation chain (line by line at minimum)
- The new monthly rent
- The arrears calculation (monthly difference × 3 maximum)
- The bank account for payment
Our tenant notification letter template is adaptable to the retroactive case.
3. Send by registered mail with acknowledgement of receipt
Sending by AR proves the date of the request, which sets the 3-month retroactivity window. Without AR, in case of dispute, the tenant may contest the date of the request — and shift the window in their favour.
4. Propose an instalment plan (amicable option)
Even limited to 3 months, the recovery — on top of the increase in the current rent — can weigh on the tenant’s budget. Many ask for an instalment plan over 3 to 6 months. Accepting this arrangement preserves the rental relationship and avoids judicial proceedings. To be formalised in writing (a simple email exchange suffices).
5. Seize the justice of the peace in case of refusal
If the tenant refuses to regularise, the procedure before the justice of the peace is free (fiscal stamp ~50 €) and fast (hearing within 4 to 8 weeks).
Risks and errors to avoid
| Frequent error | Consequence | How to avoid it |
|---|---|---|
| Notify by simple email | Date of the request contestable, lost months | Always registered mail with AR |
| Claim more than 3 months | Tenant can refuse the excess part | Limit to 3 months as a rule |
| Calculate without chain (initial rent × last index) | Underestimates the rent due | Use the calculator |
| Forget the EPC coefficient (Bxl/Fla) | Overestimates the recovery, contestation | Check the region and lease date |
| Unregistered lease | No right to indexation, even retroactive | Regularise first (cf. unregistered lease guide) |
| Landlord who has accepted multiple times without indexing | Tacit waiver argument (rare but possible) | Notify without delay |
Special case: lease recently transferred / change of owner
If the property has been sold during the lease, the new landlord inherits the lease as is, including the right to arrears for the 3 months preceding the month of their request — whatever the attitude of the previous landlord. The 3-month window runs from the first written request by the new landlord, not from the purchase date.
To go further
- Indexation after 5 years: complete guide — the flagship article on long recovery
- Calculate the exact arrears — applies the chain automatically
- Notification template — adaptable form letter
- Tenant letter form — mandatory elements
- Tenant refusal: remedies — procedure if disagreement
- Indexation cap — EPC coefficients by region
- Methodology — legal sources (art. 1728bis Civil Code, art. 34 Vlaams Woninghuurdecreet)
The 3-month rule is harsh, but it has a double virtue: it forces fast action, and it only touches the past. A landlord who sends the request as soon as the omission is discovered limits the loss to the strict minimum and preserves the full indexed rent for the future.
Frequently asked questions
What is the maximum retroactive duration of a rent indexation in Belgium? + −
Three months. The indexation request only has retroactive effect for the three months preceding the month of the request (article 1728bis, §1 of the Civil Code; confirmed in Wallonia and Brussels by the regional authorities and in Flanders by article 34 of the Vlaams Woninghuurdecreet). Whatever the duration of the omission — 2, 5 or 10 years — you can only claim those 3 months of arrears. The rest is definitively lost. The future rent, however, remains fully due at its full indexed value.
From when does the 3-month retroactivity run? + −
From the date of the written indexation request sent to the tenant. Concretely: if you send your request in May 2026, you can claim the indexation arrears for February, March and April 2026 — the three months preceding the month of the request. Before that window, it is legally lost. The signature date or the anniversary date do not count — only the date of the written request sets the window. Registered mail with acknowledgement of receipt remains advisable as proof of that date.
Can the tenant refuse to pay the 3 months of arrears? + −
No, if the request is regular (registered lease, correct calculation, exact Statbel indexes). The tenant can however spread the payment over a few months to absorb the shock — this is a common amicable agreement. Failing agreement, the landlord can seize the justice of the peace to obtain an executory title and, if applicable, a payment schedule imposed by the judge.
Should one calculate a single indexation or a chain of annual indexations? + −
The chain of cascading indexations is useful pedagogically: each year is calculated on the previous year's indexed rent. But mathematically, the result is identical to the direct formula: base rent × new index / starting index. That full formula is what determines the future rent after the request — the omission never reduces the future rent. For the arrears, however, only the 3 months preceding the month of the request are recoverable.
Can the landlord choose to claim only part of the arrears? + −
Yes. The landlord may claim an amount less than the full 3 months (as a goodwill gesture, to preserve the relationship, or to facilitate an amicable agreement). This partial waiver does not create precedent: the landlord can demand full future indexations again. But it must be formalised in writing to avoid later misunderstandings.
If the tenant has spontaneously paid the indexed rent, can they claim the overpayment back? + −
No, in principle. Once the indexed rent is spontaneously paid (without reservation), valid payment exists and the tenant cannot demand reimbursement, except by proving a defect of consent (substantial error, fraud). In practice, spontaneously paying an unnotified indexed rent amounts to tacit acceptance — the tenant loses their leverage to contest.
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).