On 20 March 2026, the Belgian Court of Cassation issued a ruling (C.24.0122.F) that simplifies the process for commercial tenants who wish to contest an indexation clause applied beyond the statutory cap. The registered letter, long regarded as a mandatory preliminary step, is no longer a condition of admissibility for bringing a court claim.
Here is what changes in practice, and where uncertainties remain.
The statutory framework: Art. 1728quater of the old Civil Code
In commercial leases, the indexation cap is set by Art. 1728bis of the old Civil Code. Any indexation clause exceeding this cap is subject to the reimbursement regime provided by Art. 1728quater §1: amounts unduly received must be returned.
This mandatory regime operates as a coherent three-step procedural block:
- The registered letter: the tenant notifies the landlord by registered letter that they are claiming reimbursement of unduly received indexation amounts.
- The restitution window: only amounts that fell due and were paid within the five years preceding the dispatch of the registered letter can be claimed. The registered letter thus retroactively defines the extent of the debt.
- The action deadline: from the date of the registered letter, the tenant has one year to bring the matter before a court (Art. 2273 para. 2 of the old Civil Code), failing which the claim is time-barred.
This cascading mechanism, in which the registered letter plays a central role, is mandatory: the parties cannot contract out of it.
What the Court of Cassation said in 2013
The ruling of 18 November 2013 (C.12.0442.F) concerned a primary residence lease governed by the Act of 20 February 1991. The question was whether a summons to conciliation could substitute for the registered letter required under a similar reimbursement regime in that Act.
The answer was no. A summons to conciliation does not produce the two cumulative effects of a registered letter: informing the landlord, on one hand, and starting the one-year limitation period, on the other. Two separate instruments cannot be combined to replicate the effect of a single act.
That ruling was often read as confirming that the registered letter was irreplaceable, not only for triggering the limitation period, but also as a gateway condition for accessing the courts.
What the 2026 ruling changes
The question put to the Court on 20 March 2026 was different: is a registered letter a condition of admissibility for a court claim seeking reimbursement of unduly received indexation in a commercial lease?
The answer is no. This is not a reversal of the 2013 position, nor a contradiction: the two rulings address distinct questions.
The contribution of 2026 is the opening of an autonomous alternative route. The tenant can go directly to court without first sending a registered letter. In that case, they can recover amounts paid in the five years preceding the judicial claim.
The registered letter therefore remains an option, not an obligation. It retains its own practical value: it starts the one-year limitation period and can, in certain situations, allow recovery of amounts over a defined period before any proceedings are launched. But it is no longer a prerequisite for accessing the courts.
Two uncertainty zones to flag honestly
The ruling opens a route, but two points remain unresolved.
(a) The limitation period for direct action without a registered letter
The one-year period in Art. 2273 para. 2 of the old Civil Code runs from the registered letter. Without a registered letter, what limitation period applies to the direct action?
The answer depends on the applicable general limitation law. Since 1 January 2023, the new Belgian Civil Code reduced the general limitation period from ten years to five years. For situations that arose before that date, the old Civil Code provided for ten years. The 2026 ruling does not resolve this question, and the period that actually applies in a given situation will depend on the circumstances and future case law.
This matters in practice: the registered letter, by triggering a one-year window, also creates a constraint. A direct action without a registered letter could in principle be brought within a much longer period, but that flexibility is precisely what generates uncertainty for both parties.
(b) Scope limited to commercial leases
The ruling of 20 March 2026 was decided in the context of a commercial lease. Whether it applies to other lease types, in particular residential leases in Wallonia governed by the Decree of 15 March 2018, is a separate question that the ruling does not address.
The Walloon Decree of 15 March 2018 contains its own indexation and reimbursement regime (Art. 27 §5), with its own procedural rules. Property managers operating in Wallonia in the residential sector cannot directly apply the reasoning of the 2026 ruling to their residential leases without further analysis.
Practical implications for property managers
The ruling shifts the balance: previously, a landlord applying a questionable indexation could rely on procedural complexity to deter challenges. Now, a commercial tenant can bring a restitution claim without the preliminary registered-letter step.
In concrete terms, an indexation calculated on a wrong basis, an unauthorised index, or a cap that was not properly respected over several years can give rise to a court claim covering five years of overpayments.
For property management agencies handling commercial leases, the risk sits at two levels:
- The right formula: indexation must strictly respect the cap set by Art. 1728bis. Any error in the formula or the reference index exposes the landlord to a restitution claim.
- The paper trail: if a claim is brought, the landlord must be able to demonstrate how each indexation was calculated, when it was applied, and on what basis. Without a record, building that case is difficult.
SEIDO calculates indexation automatically at the correct formula and keeps a timestamped history of every rent and every payment per property. If a tenant makes a retroactive claim, the agency has a complete, dated record to respond point by point. For more on the mechanics of indexation calculation, see the article Rent indexation calculator: Belgium.
Summary of key points
- Ruling Cass. 20/03/2026 (C.24.0122.F): in commercial leases, a tenant can claim reimbursement of unduly received indexation without first sending a registered letter.
- Direct route: the tenant can go directly to court and recover amounts paid in the five years preceding the judicial claim.
- The registered letter remains useful: it starts the one-year limitation period (Art. 2273 para. 2) and retains its own logic within the three-step procedural block of Art. 1728quater.
- 2013 vs 2026: no contradiction, different questions. In 2013 the issue was whether something could substitute for the registered letter; in 2026 the issue was whether the registered letter was a mandatory gateway to the courts.
- Uncertainty (a): limitation period for direct action without a registered letter, not settled by the ruling.
- Uncertainty (b): scope limited to commercial leases; applicability to residential leases, particularly in Wallonia, requires separate analysis.
Sources and references
- Old Belgian Civil Code, Art. 1728bis (indexation cap in commercial leases)
- Old Belgian Civil Code, Art. 1728quater §1 (reimbursement regime for unduly received amounts)
- Old Belgian Civil Code, Art. 2273 para. 2 (one-year period to bring a court claim from the registered letter)
- Court of Cassation, ruling of 20 March 2026 (C.24.0122.F)
- Court of Cassation, ruling of 18 November 2013 (C.12.0442.F)
- New Belgian Civil Code (general five-year limitation period, in force since 1 January 2023)
- Walloon Decree of 15 March 2018 on residential leases, Art. 27 §5