Indexation regularisation: procedure
Error, omission, resolved disagreement: how to regularise a rent indexation in Belgium. Step-by-step procedure for landlord and tenant.
Brief answer
Regularising a rent indexation in Belgium consists of restoring legal and financial compliance after an irregularity: omission, calculation error, resolved disagreement. The standard procedure is amicable (notification + agreement on the amount + payment in one go or staggered). In case of impasse, the justice of the peace decides in 2-3 months. For the past, regularisation remains capped at the 3 months preceding the month of the request (article 1728bis, §1 of the Civil Code) — beyond that, the arrears are definitively lost. For the future, however, the fully indexed rent applies from the request.
Fastest method: immediate recalculation via the calculator, notification by registered mail with instalment proposal, written agreement on the schedule. 90% of cases settle this way without justice.
When do we speak of « regularisation »?
The term « regularisation » covers several distinct situations which have in common the aim of restoring a situation compliant with law and contract:
Situation 1: Retroactive omission
The landlord notices they have not indexed for 1, 3 or 5 years. See our forgotten indexation guide for the diagnosis and complete action plan.
Situation 2: Calculation error to correct
Indexation has been notified but the calculation is wrong (incorrect index, omission of the regional EPC coefficient, etc.). The landlord or the tenant detects the error and wants to restore the right amount.
Situation 3: Disagreement resolved after contestation
A dispute is resolved in favour of the landlord (the tenant had wrongly contested). The arrears due during the disagreement period must be restored.
Situation 4: Lease newly registered after omission
The lease was not registered (so no indexation possible). The landlord regularises the registration and then wants to recover what can be.
Situation 5: Calculation to revise after sale of the property
The new owner notes past unapplied indexations. They regularise for the last 3 months (legally limited retroactivity).
The golden rule: the 3-month cap
Whatever the regularisation motive, the 3-month cap systematically applies: the indexation request only has retroactive effect for the three months preceding the month of the request (article 1728bis, §1 of the Civil Code). This means:
- Beyond 3 months, arrears are definitively lost
- The period is counted back from the month of the request (written notification received by the tenant)
- The landlord can never claim more than 3 monthly differences for the past
- For the future, the request applies in full: the fully indexed rent (complete formula from the base rent) is due from notification
This is the structuring rule of any regularisation. For detail, see our retroactive indexation guide.
Amicable procedure: 4 steps
Amicable regularisation is the preferred route — cheaper, faster, preserves the rental relationship.
Step 1: Calculate what is due
Use the calculator to rebuild the chain of theoretical indexations from the lease signature. Identify:
- The theoretical new monthly rent
- The monthly difference compared to the rent effectively paid
- The total recoverable arrears (difference × 3 months maximum)
Step 2: Draft the notification letter
The letter must contain:
- Complete identification of the parties (landlord and tenant)
- Lease reference (signature date)
- Calculation detail (base index, new index, formula, possible EPC coefficient)
- Applicable new monthly rent
- Amount of regularisable arrears (3 months maximum)
- Instalment proposal (if applicable, as commercial gesture)
- Bank account for payment
- Handwritten or qualified electronic signature
Our notification template covers all these elements.
Step 3: Send by registered mail with acknowledgement of receipt
The registered mail fixes the date of receipt, the legal starting point of regularisation. See our registered mail mandatory? guide for alternatives (eBox, email).
Step 4: Formalise the agreement in writing
If the tenant accepts the regularisation and the possible instalment plan, formalise by a simple email exchange or a written addendum (depending on stakes):
« Madam/Sir tenant, I confirm your acceptance of the indexation at the amount of [X] €/month from [date], as well as the payment of arrears of [Y] € over [N] monthly instalments of [Z] € from [date]. The whole counts as agreement act. »
This is the document that will protect landlord and tenant in case of subsequent dispute.
Classic instalment modalities
For a recovery of several hundred euros, the amicable instalment plan is standard practice. Three frequent modalities:
| Modality | Characteristics | When to use |
|---|---|---|
| One-time payment | Total at the next due date | Small amounts (< 200 €) |
| Short instalment (3 months) | 3 equal payments on top of current rent | Medium amounts (200-1,500 €) |
| Long instalment (6 months) | 6 payments on top of current rent | Large recoveries (> 1,500 €) |
| Very long instalment (12 months) | Exceptionally | Proven financial difficulty |
The instalment plan is never an obligation for the landlord, but it is almost always the most rational decision to preserve the relationship and avoid procedural costs.
Judicial procedure: if amicable fails
If the tenant refuses the amicable regularisation despite a compliant request, the landlord can seize the justice of the peace of the dwelling’s commune. Procedure:
- Prior formal notice by registered mail (15 days to regularise)
- Without response, filing of a petition at the registry (fiscal stamp ~50 €)
- Hearing within 4 to 8 weeks
- Judgment within 1-4 weeks following
- Forced execution by bailiff if the tenant doesn’t pay (1-2 additional months)
See our legal contestation guide for the detailed procedure.
Estimated total cost:
- Justice of the peace: 50 € stamp + ~100-300 € bailiff if execution = ~150-350 €
- If lawyer (not mandatory): + 300-1,000 €
These costs are generally borne by the tenant if the landlord prevails.
Worked practical cases
Case A: Simple regularisation without contestation
- Initial rent 750 € since 2020, never indexed
- Theoretical rent in 2026: 850 € (~+100 €/month after 6 years)
- Notification on 15 May 2026: 850 €/month from June 2026
- Regularisable arrears: 100 € × 3 months = 300 €
- Tenant acceptance with 3-month instalment: 100 €/month extra
- Total regularisation: 4 months (June 2026 → September 2026)
Case B: Regularisation after calculation error
- Landlord notified in Sept 2025 an indexation at 950 € (instead of correct 940 €)
- The tenant detects the error in Jan 2026 (after 4 months of overpayment)
- Regularisation: refund of overpayment = 10 € × 4 = 40 €
- Modality: deduction on next month’s rent or direct transfer
- Delay: generally < 1 week
Case C: Regularisation after judicial disagreement
- Landlord notified indexation contested by tenant (ground: « rent too expensive »)
- Justice of the peace sides with the landlord (invalid ground)
- Regularisation: full payment of arrears + costs (50 € fiscal stamp) borne by the tenant
- Delay: per judge’s decision, generally 1 month to pay
Case D: Regularisation after sale of the property
- Initial landlord did not index for 4 years
- Sale in March 2026, new owner detects the omission
- Notification to tenant on 1 April 2026
- Regularisation: last 3 months × monthly difference = recoverable
- Everything before those 3 months: lost (limited retroactivity, art. 1728bis, §1 CC)
- Liability action against former owner if recent sale with warranty
Frequent errors during a regularisation
| Error | Consequence |
|---|---|
| Claim more than 3 months of arrears | Tenant can legitimately refuse the excess part |
| Calculate by « simple » chain (initial rent × last index) | Underestimates the recovery |
| Forget the regional EPC coefficient | Invalid calculation, contestation |
| Notify without registered mail | Date of receipt contestable, lost months |
| Refuse all instalment plans systematically | Tips the file into court |
| Not formalise the agreement in writing | Potential dispute on agreement content |
| Unilaterally offset against the rental guarantee | Illegal without tenant agreement or judicial decision |
To go further
- I forgot to index — diagnosis and action plan
- Retroactive indexation: rules — retroactivity limited to 3 months
- Indexation after 5 years — long case
- Calculate the official indexation — reference tool
- Notification template — form letter
- Formal notice template — preliminary judicial step
- Contestation before the justice of the peace — if dispute
- Registered mail mandatory? — accepted formats
- Methodology — legal sources
Regularising an indexation means going back within the limits of what the law allows. The amicable route remains almost always the most rational — the justice of the peace is a safety net, not an objective.
Frequently asked questions
What does 'regularising' an indexation mean? + −
It means restoring the legal and financial compliance of the indexation after an irregularity: landlord's omission, calculation error, resolved disagreement, late payment of arrears. Regularisation can be amicable (direct agreement between landlord and tenant) or judicial (judgment of the justice of the peace). It supposes a calculation of the exact amount due and an agreed payment schedule.
How long does it take to regularise an indexation? + −
From a few days (immediate payment of arrears) to several months (amicable instalment plan over 3-6 months). If the judicial route is necessary, count 2 to 3 months between the petition and the decision, plus 1-2 months for forced execution by bailiff. The amicable route remains by far the fastest and least costly.
Can the landlord impose a regularisation in one go? + −
Legally yes, the tenant is required to pay the arrears in full upon receipt. In practice, many landlords accept an amicable instalment plan to preserve the relationship and avoid a judicial procedure. Refusing an instalment plan remains the landlord's right, but must be clearly formalised in writing to avoid any misunderstanding.
Does regularisation erase the landlord's past errors? + −
No. For the past, regularisation only covers financial recovery within the limit of the 3 months preceding the month of the request (article 1728bis, §1 of the Civil Code). Anything beyond those 3 months is definitively unrecoverable. For the future, however, the request applies in full: the fully indexed rent is due from notification. Regularising = recovering what can legally be, not rewriting history.
Is a lease addendum required to formalise the regularisation? + −
Not necessarily. For a simple arrears recovery, a notification letter followed by payment (in one go or staggered) is sufficient. For a lasting change in modalities (for example, future suspension of indexation for 1 year as commercial offset), an addendum signed by both parties is strongly recommended for traceability.
Can the tenant refuse an amicable regularisation and prefer the judicial route? + −
Yes, that is their right. If the tenant contests on a valid ground (unregistered lease, erroneous calculation, prescription exceeded), they have every interest in letting the landlord seize the justice of the peace where they will be dismissed. If the contestation is invalid, the tenant risks conviction with costs in addition to arrears. Amicable regularisation is generally the reasonable compromise.
Calculate your indexation
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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).