A lease can be amended at any time by mutual agreement
Yes, a lease can be amended during its term by mutual agreement. Both parties must sign a written addendum and register it with the FPS Finance within 2 months. No party can unilaterally impose a modification. Certain mandatory provisions (minimum duration, deposit caps, tenant rights) cannot be waived even by agreement.
The principle of contractual freedom allows parties to modify their agreement at any point during its term. This flexibility is important because circumstances change over time — a new job, a growing family, or building renovations may necessitate adjustments to the lease terms.
The key requirement is that the modification is consensual. If one party proposes a change and the other refuses, the existing lease terms remain in force. Neither the landlord nor the tenant can force the other to accept modifications.
Common lease modifications during the term
| Modification | Addendum needed | Registration |
|---|---|---|
| Adding or removing a co-tenant | Yes | Yes (free) |
| Changing the rent amount | Yes | Yes (free) |
| Authorising a pet | Yes | Yes (free) |
| Changing payment method | Yes (recommended) | Yes (free) |
| Modifying termination conditions | Yes (within legal limits) | Yes (free) |
| Adding a parking space | Yes | Yes (may have fees) |
Each modification requires a clear addendum that:
- References the original lease
- Describes the change precisely
- States the effective date
- Bears all parties’ signatures
Keep all addenda together with the original lease in a safe place. Each modification should reference the previous state and the new terms. A well-documented lease history prevents disputes and simplifies any future legal proceedings.
When unilateral modification is possible
In exceptional cases, a modification can be imposed without the other party’s agreement:
- Rent revision by the justice of the peace — at the end of each three-year period, either party can request a rent revision if circumstances have changed significantly (more than 20% change in normal rental value)
- Works imposed by law — if regulations require safety or energy improvements, these may affect lease terms
- Court order — a judge may modify lease conditions in case of changed circumstances (theory of imprevisibility, applied restrictively)
Outside these exceptions, any change requires bilateral consent. A landlord who unilaterally modifies the lease (e.g. increasing the rent without agreement) is acting in breach of contract and the tenant can challenge the change before the justice of the peace.
Regional specifics
Brussels-Capital Region
The ordinance of 27 July 2017 applies to modifications of Brussels residential leases. All addenda must respect the mandatory provisions of the ordinance. Brussels courts actively verify that modifications do not circumvent tenant protections.
Walloon Region
The decree of 15 March 2018 similarly requires that modifications comply with mandatory provisions. Wallonia applies the same principles of contractual freedom within legal boundaries.
Flemish Region
The Vlaams Woninghuurdecreet of 9 November 2018 follows comparable rules. Flanders has specific provisions for colocation modifications, allowing co-tenant rotation with limited landlord intervention.
Civil Code, art. 1134 (contractual freedom). Regional housing legislation setting mandatory provisions that cannot be waived. Rent revision: regional provisions at the end of each three-year period.