A written modification of the original lease
A lease addendum (avenant au bail) is a written document that modifies one or more clauses of the original lease without replacing it. It must be signed by all parties and registered with the SPF Finances, just like the original lease. The original lease remains in force for all non-modified clauses.
An addendum is the legally correct way to modify an existing lease. Verbal agreements to change lease terms are generally not enforceable for residential leases (which must be in writing).
Common situations requiring an addendum:
- Change of rent amount (outside of legal indexation)
- Extension or reduction of the lease duration
- Addition or departure of a co-tenant
- Change of payment terms or charges
- Modification of the permitted use of the property
When to use an addendum
| Situation | Addendum needed? |
|---|---|
| Annual rent indexation | No — automatic by law |
| Rent increase beyond indexation | Yes |
| Adding a co-tenant | Yes |
| Removing a co-tenant | Yes |
| Changing the deposit amount | Yes |
| Extending the lease duration | Yes |
| Authorising pets | Yes |
| Removing the solidarity clause | Yes |
| Changing landlord details (sale) | Yes (or new lease) |
An addendum is not appropriate for fundamental changes to the lease (e.g., changing the type of lease from residential to commercial). In such cases, a new lease should be drafted.
BailBelgique generates addenda that are automatically linked to the original lease, ensuring consistency and compliance with registration requirements.
How to draft a lease addendum
A proper addendum should include:
- Reference to the original lease (date, registration number, parties)
- Identification of the clause(s) being modified (quote the original text)
- New wording of the modified clause(s)
- Effective date of the modification
- Signatures of all parties (original signatories of the lease)
- Statement that all other clauses remain unchanged
The addendum must be drafted in the same number of copies as the original lease (one per party plus one for registration).
Registration: the addendum must be registered with the SPF Finances within 2 months of signature, just like the original lease. Registration is free for residential leases. Failure to register the addendum has the same consequences as non-registration of the lease.
Regional specifics
Brussels-Capital Region
Brussels does not impose a specific format for addenda. The addendum must comply with the language requirements of the original lease and be registered.
Walloon Region
Wallonia requires the addendum to be in writing and registered. The information obligations that apply to the original lease also apply to significant addenda.
Flemish Region
In Flanders, if the original lease used the mandatory template, the addendum should reference the template and clearly identify which sections are being modified. Registration is mandatory.
Belgian Civil Code (freedom of contract) + Registration Code + Regional housing legislation — Addenda must be written, signed by all parties and registered.