Indexation and co-tenancy: specific rules

Brief answer

Indexation in co-tenancy follows two logics depending on the legal regime: joint single lease = one annual indexation for the total rent, on the anniversary date; individual leases per room = one indexation per lease, on each individual anniversary date. The co-tenancy pact (Brussels since 2018, Wallonia since 2018) frames solidarity and distribution but does not modify the federal indexation mechanics.

Three possible co-tenancy regimes

1. Joint single lease (the most frequent)

Characteristics:

• A single contract signed by all co-tenants at the same time
• Passive solidarity: each co-tenant is bound for the full payment
• Single anniversary date = lease signature date
• Global rent negotiated between landlord and co-tenant group

Indexation: a single annual notification to the group (or to a designated representative), calculation on the total rent. The distribution between co-tenants is defined by their internal pact (or internal rule) — the landlord does not get involved.

2. Individual leases per room

Characteristics:

• Each co-tenant signs their own lease for their room + access to common areas
• No solidarity between co-tenants (each only pays their share)
• Individual anniversary dates (according to respective signature dates)
• Per-room rent negotiated individually

Indexation: one notification per lease, on each individual anniversary date. If 4 co-tenants arrived at 6-month intervals, the landlord sends 4 distinct notifications per year (spread over the year).

This is the classic regime of the student room and large houses divided into independent rooms.

3. Co-tenancy pact (Brussels + Wallonia since 2018)

Characteristics:

• Inter-co-tenant document attached to the joint single lease
• Defines: internal rules (who pays what, who sleeps where, cleaning, charges), arrival/departure procedure for a new co-tenant, individual notice without breaking the collective lease
• In the Brussels Housing Code (art. 261/1 et seq. since 2018)
• In Wallonia, framed by the decree of 15 March 2018

Indexation: no modification of federal mechanics. The pact, however, can specify the distribution of the indexation between co-tenants (equal, prorated by room, etc.). It is an internal rule with no effect on the landlord.

Which regime for which situation?

Typical situation	Recommended regime	Why
Unmarried couple	Joint single lease	Simplicity, natural solidarity
3-4 co-tenant friends	Single lease + pact	Individual notice possible
Student room (4 rooms)	Individual leases	Independent annual renewal
Coliving house 6+	Individual leases	High turnover, easier management
Blended family	Single lease	Classic couple regime

Indexation: what changes by regime

Case A: Joint single lease, 4 co-tenants, total rent 1,600 €

• Initial rent: 1,600 €/month (400 € per head conventionally)
• 1^st indexation: +40 €/month → total rent 1,640 €/month
• Distribution according to internal pact:
  • Equal: +10 €/head
  • Prorated by m² (rooms 12 / 14 / 16 / 20 m² = 62 m²): 7.74 € / 9.03 € / 10.32 € / 12.90 €
• The landlord claims 1,640 € from any one of the co-tenants (solidarity)

Case B: Individual leases, 4 rooms, different rents

• Room 1: 350 €/month (signed 1 Sept 2024) → 1st indexation 1 Sept 2025 (+9 €)
• Room 2: 380 €/month (signed 1 Dec 2024) → 1st indexation 1 Dec 2025 (+10 €)
• Room 3: 420 €/month (signed 1 April 2025) → 1st indexation 1 April 2026 (+11 €)
• Room 4: 360 €/month (signed 1 June 2025) → 1st indexation 1 June 2026 (+9 €)

The landlord sends 4 distinct notifications spread over 9 months. Each co-tenant only pays their own indexation.

Case C: Brussels co-tenancy pact, gradual switch

• Single lease signed Sept 2023 by 3 co-tenants (Alice, Bob, Carla)
• Total rent 1,500 €/month, indexation on the September anniversary date
• Carla leaves March 2026 (individual notice per pact) — David replaces her
• The main lease continues with Alice + Bob + David (signed addendum)
• The next indexation (Sept 2026) remains calculated on the original lease of Sept 2023

The change of co-tenant during the lease does not reset anything — the indexation chain follows its normal progression.

Notification procedure in co-tenancy

For a joint single lease

1. A single letter addressed to all co-tenants (or to the representative designated in the pact)
2. Registered mail sent to the property’s address, or in person during a visit
3. Calculation on the total rent via the calculator
4. The internal distribution is managed by the co-tenants (the landlord does not decide)

For individual leases per room

1. One letter per co-tenant, on their own anniversary date
2. Registered mail addressed to each at their notification address (often the property’s address)
3. Individual calculation for each room via the calculator
4. The landlord keeps an in-house calendar of individual anniversary dates

The letter template remains the same: see our notification template, to be duplicated if multiple leases.

Special cases and frequent traps

Trap 1: Believing that a co-tenant change cancels the indexation

Reality: the indexation chain follows the lease, not the persons. A new co-tenant during the lease does not reset anything. They inherit the situation as it is, indexed rent included.

Trap 2: Indexing each co-tenant individually in a single lease

Error: sending 3 distinct letters for a single lease. Multiple indexations are null (1^st = valid, following = null). A single notification suffices for the whole.

Trap 3: Claiming from one co-tenant alone, without informing the others

Risk: internal conflict between co-tenants, impression of inequality. Good practice: formally notify all, even if only one effectively pays. Safeguard in case of subsequent dispute.

Trap 4: Confusing passive solidarity and automatic division

Passive solidarity = the landlord can claim 100% from any co-tenant. This does not mean the landlord must wait for the co-tenants to divide among themselves. If 200 € are missing on the rent, the landlord claims these 200 € from one of them, who will then turn against the others.

Trap 5: Forgetting the regional EPC coefficient

As for all residential leases, the EPC coefficient applies in Brussels (leases prior to 14/10/2022) and Flanders. See our EPC classes guide and calculator which automatically applies the right coefficient.

Subletting between co-tenants: distinct regime

If a co-tenant sublets their room to a third party (with the landlord’s authorisation), a second sublease is created between the sub-landlord (= the co-tenant) and the sub-tenant:

• Indexation of this secondary lease according to classic rules
• But cap: the sublet rent cannot exceed the share of rent paid by the sub-landlord to the main landlord
• If the indexation of the main lease increases the sub-landlord’s share, the latter can index their own sublease in the same proportion

This complex structure is rare in practice but legally possible. A specific subletting guide goes beyond the scope of this article.

To go further

• 9-year lease and indexation — underlying classic regime
• Short-term lease — frequent in student rooms
• When can one index a rent? — temporal rules
• How to calculate an indexation — step-by-step formula
• Calculate my indexation — official tool
• Notification template — form letter
• EPC coefficient by class — to apply in Bxl/Fla
• Unregistered lease — common sanction
• Methodology — legal sources

Co-tenancy adds an organisational dimension to indexation, but does not modify the law. The golden rule remains: one lease = one annual indexation = one calculation on the rent of that lease.