The anniversary date is the first day of the signing month
When a lease is signed mid-month (e.g., on 15 March), the indexation anniversary date is the 1st of that month (1 March). The health index used for calculation is from the month preceding the anniversary date (February). This rule simplifies the calculation and ensures consistency.
Belgian law provides a clear rule for mid-month lease signings: the indexation date is always rounded to the first day of the month. This applies to all types of residential leases and is codified in the regional housing legislation.
The rent indexation formula remains the same regardless of when in the month the lease was signed:
Indexed rent = (base rent x new health index) / starting health index
The starting index is the health index of the month preceding the lease signature month. This reference point remains fixed throughout the lease.
Step-by-step calculation
Here is how to calculate indexation for a mid-month lease:
- Identify the anniversary date: first day of the month the lease was signed
- Find the starting index: health index of the month before the signing month
- Find the new index: health index of the month before the current anniversary date
- Apply the formula: (base rent x new index) / starting index
Example:
- Lease signed on 15 March 2024, base rent: 850 EUR
- Anniversary date: 1 March each year
- Starting index: February 2024 health index
- New index (for 2025): February 2025 health index
Use the BailBelgique indexation calculator to automatically compute the correct indexed rent. Simply enter your lease signing date and base rent — the tool handles the mid-month calculation automatically.
Common mistakes to avoid
Several errors frequently occur when calculating indexation for mid-month leases:
| Mistake | Correct approach |
|---|---|
| Using the signing date as anniversary | Use the 1st of the signing month |
| Using the index of the anniversary month | Use the index of the month before the anniversary |
| Indexing from the move-in date | Always index from the signing date |
| Applying indexation without a registered lease | Registration is required before indexing |
It is important to remember that rent indexation is not automatic — the landlord must formally request it. The request can only be made after the lease anniversary date, and it cannot be applied retroactively beyond 3 months.
If your lease is not registered with the FPS Finance, you cannot legally index the rent. Ensure your lease is registered before sending an indexation notice to your tenant.
Regional specifics
Brussels-Capital Region
The Brussels Housing Code follows the federal indexation rules for mid-month leases. The anniversary date is the 1st of the signing month. Brussels has additional restrictions on indexation for properties with low energy performance (EPC D, E, F, G).
Wallonia
Wallonia applies the same principle. The Walloon Residential Lease Decree confirms that the indexation date for mid-month leases is the first of the month. Wallonia has also introduced energy-based indexation limits.
Flanders
The Flemish Housing Rental Decree uses the same calculation method. Flanders temporarily limited indexation for energy-inefficient homes (EPC D, E, F) and this may be extended.
Belgian Civil Code, art. 1728bis — Rent indexation rules. Regional housing legislation confirms the first-of-the-month convention for mid-month lease signings.