I forgot to index for years: what to do?

Brief answer

If you discover that you forgot to index your rent, the rule is simple in Belgium: you can recover the last 3 months maximum (the three months preceding the month of the request), whatever the duration of omission (2, 5 or 10 years). Everything before that is legally lost — but for the future, the rent moves to the fully indexed amount. Acting fast is essential: each month of additional delay costs one month of potential recovery.

30-second diagnosis: did you really forget?

Before panicking, check three things:

1. Have you sent a formal letter (simple letter, registered mail or explicit email) to the tenant in previous years? A simple oral mention does not count — a written document is required. A text message or an email qualifies (Wallonia expressly admits it), provided you keep proof of it.
2. Is the tenant still paying the initial rent (the one in the lease) or a slightly higher amount? If they pay a higher amount, there may have been an indexation you forgot to archive.
3. Do you have an agency or syndic in charge of management? Check with them before concluding to an omission.

If after these three checks you confirm the omission, move immediately to action.

Why act right now?

The retroactivity of an indexation is capped at the three months preceding the month of the request by article 1728bis, §1 of the Belgian Civil Code. These 3 months are counted back from the month of your written request; registered mail proves that date.

Concretely: as long as you do not notify, you lose recovery every month. If you notify in May 2026, you recover the arrears from February to April 2026. If you notify in June 2026, you recover those from March to May 2026 — one month lost for nothing.

For the detail of this retroactivity limit, see our retroactive indexation guide.

4-step action plan

Step 1: Calculate what should have been collected

Use the calculator. Enter the lease signature date, its entry-into-force date and the date of the latest anniversary of that entry into force (it sets the new index, not the date you send your letter). The calculator directly applies the official formula (base rent × new index ÷ starting index): there is no need to recalculate year by year.

Note two figures:

• The theoretical new monthly rent (to apply for the future)
• The monthly difference compared to the rent currently paid

Step 2: Calculate the recoverable arrears

Arrears = (Indexable rent for the month concerned − Rent paid), over the 3 months preceding the month of the request

That’s all. No need to go further: retroactivity blocks at 3 months no matter what. Note: months before the latest anniversary are calculated at the rent indexable at the previous anniversary, not at the new amount.

Example: lease signed and entered into force on 1 June 2018 (Wallonia, EPC B), base rent 800 €, never indexed, request sent in June 2026. New rent from June 2026: 800 × 139.22 ÷ 106.99 = 1,040.99 €. The 3 preceding months (March, April, May 2026) fall under the 1 June 2025 anniversary: 800 × 134.54 ÷ 106.99 = 1,006.00 €, i.e. a 206.00 € difference per month and 618 € arrears maximum.

Step 3: Draft and send the letter

The letter must contain:

• Full identity of the landlord (you)
• Identity of the tenant and address of the property
• Lease reference (signature date)
• The starting index (month preceding signature in Brussels and Wallonia; month preceding entry into force in Flanders for leases concluded since 2019)
• The new index (month preceding the latest anniversary of the entry into force)
• The detailed calculation (base rent × new index ÷ starting index)
• The new monthly rent from the next payment due
• The calculation of arrears over 3 months
• The bank account for payment
• A handwritten signature

Our notification template covers these elements. No need to explain the omission — stay factual.

Send by registered mail with acknowledgement of receipt. Registered mail proves the date of your request, from which the 3 months of retroactivity are counted back.

Step 4: Anticipate the reaction and propose an instalment plan

For a recovery of several hundred euros, many tenants ask to spread the payment. Anticipating this request in the initial letter is a sign of good faith that often soothes the relationship:

« I propose an instalment plan over 3 monthly payments of 206 € on top of the current indexed rent. If you prefer another rhythm, do not hesitate to get back to me. »

Such a proposal immediately defuses the potential conflict.

Managing the relationship with the tenant

A sudden recovery is always poorly received by the tenant, even when legally founded. Three principles to preserve the relationship:

Gesture	Effect on the relationship
Notify by registered mail without amicable warning	Brutal effect, conflict guaranteed
Announce orally before sending the registered mail	Soothes, tenant warned
Propose an instalment plan from the start	Constructive climate
Explain in a few words (without apologising)	Credibility maintained
Waive part of the arrears	Appreciated gesture, to formalise

Many landlords choose to waive 1 or 2 months of arrears as a goodwill gesture: economically marginal, it is a powerful relational lever. This waiver must be formalised in writing to avoid future misunderstandings (« you waived everything, does that mean you will also waive in the future? »).

Special case: omission attributable to your agency

If you mandated a real estate agency or a syndic for rental management and they omitted the indexation, two levels of action:

1. Normal procedure vis-à-vis the tenant: notification + 3-month recovery (nothing changes)
2. Liability action against the agency for the definitive loss (periods beyond 3 months not recoverable)

The classic recourse begins with a formal notice to the agency detailing the professional fault (the omission of the indexation mission is generally explicit in the management contract) and quantifying the prejudice (sum of lost annual indexations). Many agencies spontaneously offer compensation to preserve their reputation.

Special case: omission on a lease already terminated

If you discover the omission after the tenant’s departure, two situations:

• The tenant is solvent and reachable: registered mail to their new address, same rules as for an ongoing lease (3 months retroactive). In case of refusal, justice of the peace.
• The tenant has disappeared or is insolvent: recovery becomes theoretical. Mobilise the rental guarantee first if it still exists, then coldly evaluate whether the judicial procedure is worth it given the amount.

Errors NOT to make

Error	Why it’s a problem
Try to claim more than 3 months	Tenant can legitimately refuse the excess part
Deduct arrears from the rental guarantee	Illegal without tenant agreement or judicial decision
Increase rent without formal written notification	No legal force, tenant can keep paying the old amount
Start from the last rent paid or from a wrong starting index, instead of the formula (base rent × new index ÷ starting index)	Wrong calculation, valid contestation
Forget the EPC correction factor (Brussels: leases in force before 14/10/2022, EPC E-G; Flanders: before 1/10/2022, EPC D-F or no EPC) or the Walloon special method	Overestimates the recovery, valid contestation
Stop collecting rent to apply pressure	False good idea, you remain bound to keep the property rented
Seize the justice of the peace directly without prior formal notice	An amicable attempt is strongly advised before going to the justice of the peace

To go further

• Indexation after 5 years: the complete guide — in-depth legal angle
• Retroactive indexation: rules and limits — detailed worked examples
• Calculate the exact arrears — the full formula, applied automatically
• Notification letter template — ready-to-sign PDF
• How to notify the tenant — mandatory elements
• Tenant refusal: remedies — if the recovery is contested
• The law on indexation — detailed legal sources

The omission happens to everyone. What distinguishes a good landlord from a bad one is not never to forget — it’s to act with method and tact as soon as discovered.