Unregistered lease: can it be indexed?

Answer in brief

It depends on the Region. In Wallonia (main-residence lease, since 1 September 2018) and in Brussels (where the EPB certificate must also have been provided to the tenant), an unregistered lease cannot be indexed: registration is a prerequisite for the indexation right, and the tenant can legally refuse any indexation request as long as this formality is not accomplished. In Flanders, a written lease and a written request suffice (art. 34 of the Flemish Housing Rental Decree).

The registration obligation

The residential lease must be registered within a 2-month deadline from the signature, at the registration office of the region where the property is located. This obligation is governed by articles 19 and 32 of the Code of Registration Rights.

Three principles to know:

1. Free: since the Law of 25 April 2007, registration of a residential lease is completely free (zero costs). No direct cost can therefore justify an oversight or postponement
2. Landlord’s responsibility: it is the landlord who is legally responsible for the registration, not the tenant. The commercial lease is an exception (shared responsibility), but in residential the responsibility is exclusive to the landlord
3. Competent office: registration is done at the registration office of the property’s region (Wallonia, Brussels or Flanders). Many procedures are now possible online via MyMinfin

This formality is therefore neither costly nor complex — its oversight generally results from simple administrative negligence, but its consequences on indexation are severe.

Consequences on indexation

In Wallonia and Brussels, the unregistered lease opens no indexation right for the landlord. Concretely:

• The tenant can refuse any request for indexation, present or past
• A registered letter sent to notify the indexation has no legal effect as long as the lease is not registered
• The landlord has no remedy before the justice of the peace: case law has been consistent for decades
• Even the arrears of the last 3 months (which could otherwise be claimed according to our after 5 years guide) are permanently lost
• In Flanders, by contrast, registration is not a condition to index: a written lease and a written request are enough (art. 34 of the Flemish decree)

This rule protects the tenant: as long as the landlord has not fulfilled their own administrative obligation (registration), they cannot demand indexation in their favour. It is an application of the legal principle that “no one can take advantage of their own turpitude”.

Worked example

Marc signed a lease in Liège in June 2021 for the amount of €700/month. Through negligence, he never registered it. In May 2026, he discovers he could have indexed each year and sends a registered letter requesting a catch-up.

The tenant refuses, rightfully:

• 5 years of theoretical indexation would have represented a cumulated lost income of several thousand euros over the period
• Marc finally regularises the registration in June 2026 (with fine ~€325)
• He can now request the indexation from the anniversary date of June 2026, calculated as usual from the base rent, with the starting index of the month preceding the original signature — late registration changes neither the starting index nor the anniversary
• But: the previous 5 years are permanently lost, without recourse

The administrative oversight cost Marc the equivalent of several months of rent — for a formality that takes 15 minutes online.

How to regularise a forgotten lease

Regularising an unregistered lease remains possible at any time, and the landlord has every interest in doing it quickly to open the indexation right for the future.

Practical procedure:

1. Identify the competent office based on the property’s region (information accessible on MyMinfin)
2. Present the original lease signed by both parties (or certified copies)
3. Pay any fines: flat fee of about €25 + €50 per month of delay, capped. For a 5-year forgotten lease, the fine remains reasonable
4. Recover the dated document bearing the official registration mention — this date is now opposable

Additional tax consequences: depending on the region, late registration can trigger a verification of the declared rental income for past years. If the landlord has correctly declared their income, no risk. Otherwise, fiscal regularisation must be anticipated.

Once registration is done, indexation becomes possible again for the future, without recovering the non-registration period. Late registration changes nothing to the calculation itself:

• The starting index remains that of the month preceding the original signature (Wallonia, Brussels)
• The anniversary date remains that of the original entry into force
• The indexed rent is computed from the base rent, as for any other lease

To estimate the new rent post-regularisation, use our calculator — calculation from your data.

And if the tenant blocks the late registration?

Since registration is a unilateral formality of the landlord, the tenant has legally no right to oppose it. They don’t need to sign anything for the regularisation.

However, in practice:

• The landlord needs the original lease signed by both parties for registration
• If the tenant holds their copy and refuses to cooperate, the landlord can use their own original copy (two copies are normally signed at signature)
• In case of total blockage (malicious tenant claiming to no longer have a written lease), the landlord can refer to the justice of the peace to have the existence of the lease established and proceed to registration

These situations remain rare. In 99% of cases, the landlord has their original copy and can regularise without any intervention from the tenant.

The other consequences of an unregistered lease

Beyond indexation, the unregistered lease deprives the landlord of several important legal protections:

• No certain opposable date: the lease is not opposable to third parties (property buyer, creditors, tax administration). In case of sale, the new owner is not bound to respect the lease
• Non-opposable notice: the tenant can freely terminate with a one-month notice, without having to respect legal deadlines or pay early termination compensation. This is a major risk for the landlord
• Weakened rental guarantee: legal protection on the blocked account can be challenged by the tenant
• More difficult eviction procedure: without a registered lease, the justice of the peace requires more evidence to order eviction in case of non-payment
• Tax consequences: the absence of registration can be considered an indication of non-declaration of rental income, triggering a tax audit

Administrative oversight is therefore not trivial: it weakens the entire legal position of the landlord, well beyond the sole question of indexation.

Special case: tacit lease or verbal agreement

A verbal agreement or tacit lease exists legally in Belgium — the Civil Code recognises the formation of contracts by verbal agreement. But without writing, two problems arise for indexation:

1. No written and dated base rent: the indexation formula requires a reference amount and a signature month. Without writing, these elements cannot be proven
2. No possible registration: a verbal agreement is not registered. Indexation is therefore structurally impossible

Solution: formalise the relationship by a written lease signed by both parties, indicating the actual move-in date as the reference date for the indexation calculation. Then register this lease within 2 months.

Once this regularisation is done, indexation becomes possible for the future, from the first anniversary of the new lease’s entry into force.

Going further

• Indexation after 5 years: rules and catch-up
• Tenant refusal: what to do?
• When can a rent be indexed?
• Notification letter template
• Complete calculation methodology
• Our indexation calculator — calculation from your data