Rent increase during a lease: is it possible?
Can the landlord increase the rent during a lease in Belgium? Indexation, triennial revision, tenant's agreement, and legal procedure.
- 01 The 3 mechanisms
- 02 Indexation
- 03 Triennial revision
- 04 Key takeaways
The 3 mechanisms for rent increases during the lease
| Mechanism | Initiative | Condition | Procedure |
|---|---|---|---|
| Annual indexation | Landlord | Provided for in the lease | Written notification |
| Triennial revision | Landlord or tenant | Rental value +/- 20% | Justice of the peace |
| Mutual agreement | Both parties | Written agreement | Amendment to the lease |
Any other unilateral increase is illegal. The landlord cannot increase the rent simply because the market has risen or because of additional expenses.
Rent indexation
How it works
Indexation is the most common mechanism. It adjusts the rent to the evolution of the cost of living (health index).
| Element | Rule |
|---|---|
| Frequency | Once a year (lease anniversary date) |
| Calculation basis | Health index |
| Formula | Base rent x (current index / starting index) |
| Notification | In writing (letter, email) |
| Retroactivity | Maximum 3 months |
| Condition | Provided for in the lease (mandatory in primary residence leases) |
Calculation example
A base rent of 800 EUR signed in January 2024 (index 125.00), with a current index of 128.50:
- New rent = 800 x (128.50 / 125.00) = 822.40 EUR
The landlord must notify the indexation in writing. Without notification, the tenant continues to pay the previous amount.
The triennial revision
Conditions
Every 3 years (between the 9th and 6th month before the end of each triennial period), the landlord or tenant may request a rent revision if circumstances have changed the rental value by at least 20%.
| Who can request | Reason |
|---|---|
| Landlord | Improvement works (EPC, kitchen…) |
| Tenant | Deterioration of the environment, nuisances |
| Both | Significant change in the local market |
Procedure
- Request by registered letter to the other party
- Negotiation (30 days)
- If no agreement, application to the justice of the peace within 3 months
[!important] Legal point The triennial revision is rarely granted by the judge. The 20% threshold for change in rental value is high and difficult to prove.
Key takeaways
Indexation is the only regular and predictable increase mechanism. The triennial revision is a rare tool. To set a market-adapted rent, the best time is when signing the lease.
Automate the indexation calculation with a rental management software. See our guide on setting the rent.
Frequently asked questions
-
Not freely. Three mechanisms exist: annual indexation (automatic if provided for in the lease), triennial revision (if the rental value has increased by 20%+), and mutual agreement (lease addendum). The landlord cannot impose a unilateral increase outside these cases.
-
Indexation is based on the health index. It applies once a year, on the anniversary date of the lease. The landlord must request it in writing. It is not retroactive beyond 3 months. The formula is: new rent = base rent x (current index / starting index).
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The tenant can refuse any increase not based on indexation or triennial revision. Even for triennial revision, the tenant can challenge before the justice of the peace if the requested increase is disproportionate.