Indexation of a 9-year lease in Belgium
The 9-year lease is the Belgian residential standard. How many indexations possible, how to calculate them, articulation with the 3-6-9 formula.
Brief answer
The 9-year lease is the standard residential lease contract in Belgium. Over its duration, the landlord can apply up to 8 annual indexations (one at each anniversary date from the 1st to the 8th), each calculated from the base rent with the lease’s starting index (increases accumulate year after year). The mechanism is compatible with the classic 3-6-9 tenant termination formula.
Key takeaway: 8 possible indexations, one at each anniversary date, always with the same formula: base rent × new index ÷ starting index. For the exact rent of each year over 9 years, the calculator applies this formula automatically.
The 9-year lease: Belgian residential standard
Since the law of 20 February 1991 (which became the principal residence lease in the Civil Code), the 9-year lease is the default regime in Belgium for principal residence rentals. If nothing is stipulated in the contract, this regime applies.
Three major characteristics:
- Fixed duration of 9 years from entry into force
- Termination right by the tenant at 3 and 6 years (3-6-9 formula) with 3-month notice
- Tacit renewal by 3-year periods in the absence of notice at term
Annual indexation fits within this structure: once a year, on the anniversary date of entry into force, the landlord can adapt the rent according to the official formula of article 1728bis of the Civil Code.
How many indexations are possible?
Eight indexations maximum, one per anniversary year:
| Year | Indexation possibility | Applicable rent |
|---|---|---|
| Entry into force | — | Initial rent |
| 1st anniversary | Indexation 1 | Base rent × index 1 ÷ starting index |
| 2nd anniversary | Indexation 2 | Base rent × index 2 ÷ starting index |
| 3rd anniversary | Indexation 3 | Base rent × index 3 ÷ starting index |
| 4th anniversary | Indexation 4 | Base rent × index 4 ÷ starting index |
| 5th anniversary | Indexation 5 | Base rent × index 5 ÷ starting index |
| 6th anniversary | Indexation 6 | Base rent × index 6 ÷ starting index |
| 7th anniversary | Indexation 7 | Base rent × index 7 ÷ starting index |
| 8th anniversary | Indexation 8 | Base rent × index 8 ÷ starting index |
| 9th anniversary = term | — | Same formula if the lease continues; new base rent if new lease |
Each indexation is always calculated from the base rent and the starting index (“index N” = health index of the month preceding the Nth anniversary). The cumulative effect of inflation is already contained in the new index: there is no need to apply a percentage to the previous year’s rent. For calculation details, see how to calculate indexation.
Worked example: a lease signed in 2017 at 800 €
Here is the complete chain of 8 possible indexations for a lease signed and entered into force on 1 September 2017 at 800 € (actual Statbel health indexes, base 2013; dwelling with a B certificate, hence no correction factor):
| Date | Health index August | Theoretical rent | Cumulative increase |
|---|---|---|---|
| Sept. 2017 (signature) | 105.68 | 800.00 € | — |
| Sept. 2018 (1st anniv.) | 107.55 | 814.16 € | +1.8 % |
| Sept. 2019 | 109.07 | 825.66 € | +3.2 % |
| Sept. 2020 | 110.20 | 834.22 € | +4.3 % |
| Sept. 2021 | 112.74 | 853.44 € | +6.7 % |
| Sept. 2022 | 123.68 | 936.26 € | +17.0 % |
| Sept. 2023 | 128.82 | 975.17 € | +21.9 % |
| Sept. 2024 | 132.94 | 1,006.36 € | +25.8 % |
| Sept. 2025 (8th anniv.) | 135.64 | 1,026.80 € | +28.3 % |
| Sept. 2026 (term) | — | (same formula if the lease continues) | — |
Over 9 years, the theoretical rent goes from 800 € to 1,027 € — i.e. +227 €/month cumulative. A single inflation shock (2022-2023) explains more than half of this increase.
To calculate your case, use the calculator by entering the signature date, the entry-into-force date and the target indexation date.
Articulation with the 3-6-9 formula
The 3-6-9 formula (termination possible at 3 and 6 years by the tenant) has no impact on the indexation mechanism. Three precisions:
If the tenant leaves at 3 years
The 3 indexations already applied are definitively acquired by the landlord. The rent paid during these 3 years is never refundable. The landlord recovers:
- The exit inventory (with or without damage at the tenant’s expense)
- The rental guarantee according to the usual modalities
- No additional compensation for the 6 unfulfilled years (the 9-year lease does not guarantee duration — it only guarantees the right to remain during 9 years)
If the tenant leaves at 6 years
Same: 6 indexations acquired, normal restitution of the rental guarantee.
If the lease goes to term (9 years)
8 maximum possible indexations, then new lease (with a new initial rent negotiated) or tacit renewal (3 additional years: indexation continues with the same formula, from the original base rent).
Our when to index guide details the exact calendar of annual deadlines.
Tacit renewal after 9 years
If neither party gives notice at the 9-year term (6-month notice), the lease is tacitly renewed for 3 years. During this period:
- Indexation continues at the same rhythm, at each anniversary of the entry into force, with the same formula
- It is always calculated from the lease’s base rent and the original starting index (renewal does not « reset » anything)
- The landlord retains the right to refuse renewal with 6-month notice before the 9-year term, against legal indemnity of 6 months’ rent
Comparison with other types of leases
| Lease type | Duration | Possible indexations | Rent renegotiation |
|---|---|---|---|
| 9-year lease (principal residence) | 9 years | 8 (annual) | Indexation only + exceptional triennial revision |
| Short-term lease | ≤ 3 years | Per clauses | Possible at each renewal |
| Commercial lease | 9 years | 8 (annual) | Triennial revision possible (1951 law) |
| Lifetime lease | Tenant’s lifetime | 1 per year | No renegotiation |
| Joint co-tenancy | Per contract | 1 per year | Per clauses |
The 9-year lease is the most stable for the tenant — and the least flexible for the landlord in terms of revaluation.
Indexation and triennial revision: do not confuse
Annual indexation is a written-request mechanism that follows health inflation. Triennial revision is a separate, exceptional device that allows revising the rent at 3, 6 or 9 years, on condition of proving a significant variation of the rental value (>20% by new circumstances: works, change of environment, etc.).
| Criterion | Indexation | Triennial revision |
|---|---|---|
| Frequency | Annual | At 3, 6 and 9 years |
| Mechanism | Landlord’s written request | Justice of the peace |
| Justification | None (mathematical formula) | >20% variation to prove |
| Tenant acceptance | Not required | Not required but contestable |
| Typical magnitude | 2-3% per year | 0 or 20-30% at once |
| Frequency of use | Very frequent | Very rare |
In practice, triennial revision is so rarely used (1-2% of leases) that most landlords rely only on annual indexation.
Errors specific to the 9-year lease
| Error | Consequence |
|---|---|
| Apply an inflation percentage to the previous year’s rent instead of the formula (base rent × new index ÷ starting index) | Cumulative deviations from the legal rent, contestable calculation |
| Index more than once a year (« linked to monthly inflation ») | Indexations 2-N are null, demand to recalculate |
| Believe that departure at 3 years cancels indexations | False, they are acquired |
| Neglect the indexation in the year of tenant departure | Loss of the right for that year (only the 3 months preceding the request are recoverable, art. 1728bis, §1) |
| Confuse indexation and triennial revision | Mixture of the two mechanisms, invalid calculation |
| Confuse renewal and new lease | Renewed lease: same base rent, same starting index; only a genuinely new lease starts from a new base rent and a new starting index |
To go further
- When can one index a rent? — annual calendar and frequency rules
- How to calculate an indexation — step-by-step method
- Indexation after 5 years — case of unindexed 9-year lease
- Short-term lease and indexation — alternative to the 9-year lease
- Calculate the indexation chain — applies the official formula at each anniversary
- The law on indexation — detailed legal sources
- Notification template — form letter to send each anniversary
The 9-year lease is designed for stability. Annual indexation gently compensates monetary erosion — without it, a rent set in 2017 would have lost about 22% of purchasing power by summer 2025.
Frequently asked questions
How many indexations are possible on a 9-year lease? + −
Eight. The first indexation can occur at the first anniversary date (1 year after the lease takes effect), then each following year up to and including the eighth anniversary date. Total: 8 annual indexations, each calculated from the base rent. The ninth anniversary corresponds to the normal end of the lease: if the same lease is prolonged or renewed, indexation continues at each anniversary with the same formula; only a genuinely new lease starts from a new base rent and a new starting index.
If the tenant leaves after 3 years (3-6-9 formula), do they lose past indexations? + −
No. Indexations already applied before departure are definitively acquired by the landlord. If the tenant has paid for 3 years a rent indexed each year (3 indexations), this indexed rent is fully due — the early departure gives rise to no retroactive reimbursement. Only the rental guarantee is returned according to the usual rules (exit inventory).
Can the landlord modify the rent outside indexation during the 9 years? + −
No, never during the 9-year lease. The initial rent set in the lease can only be modified by the annual indexation (on the landlord's written request) or by an addendum signed by mutual agreement with the tenant. Any unilateral increase is null. The landlord can only renegotiate the rent at the end of the lease (for a new tenant) or upon explicit renewal.
What happens to indexation upon tacit renewal after 9 years? + −
If neither party gives notice at the term, the lease is tacitly renewed for 3 years (depending on the region). Indexation then continues at the same rhythm, at each anniversary of the entry into force. It is always calculated from the lease's base rent and the original starting index; renewal does not reset anything.
Can the 9-year lease provide a degressive initial rent or a triennial revision? + −
Yes, under conditions. A triennial revision (at 3 and 6 years) of the rent is legally possible but must be justified by a significant variation of the rental value of the property (>20% in Wallonia, >20% in Brussels Region). This procedure is rare and delicate — it goes through the justice of the peace. Annual indexation, in contrast, is made on a simple written request and requires no justification.
Does a 9-year lease signed in 2018 use the same indexation rules as a lease signed in 2025? + −
The principle (formula, annual frequency, mandatory written request) is identical. The health index base makes no difference (two indexes expressed in the same base give the same ratio; our calculator uses base 2025 by convention for leases signed from 1 January 2026), but the regional EPB/EPC correction factors depend on the lease's date of entry into force (Brussels: leases that entered into force before 14/10/2022; Flanders: before 1/10/2022). Our [calculator](/en/calculator/) handles both dimensions automatically.
Calculate your indexation
Our tool automatically applies the Belgian legal formula with up-to-date Statbel indexes.
Go to the calculator →This article is informational and does not constitute legal advice. For any disputed or complex situation, consult a lawyer or the justice of the peace of the property's commune (low-cost procedure, no lawyer required).