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 on the previous indexed rent (snowball effect). The mechanism is compatible with the classic 3-6-9 tenant termination formula.
Key takeaway: 8 possible indexations, calculated in chain (cascade), at each anniversary date. For the exact calculation of an indexation chain over 9 years, the calculator automatically applies the cascade.
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 | Initial rent × index 1 |
| 2nd anniversary | Indexation 2 | Indexed rent year 1 × index 2 |
| 3rd anniversary | Indexation 3 | Indexed rent year 2 × index 3 |
| 4th anniversary | Indexation 4 | Indexed rent year 3 × index 4 |
| 5th anniversary | Indexation 5 | Indexed rent year 4 × index 5 |
| 6th anniversary | Indexation 6 | Indexed rent year 5 × index 6 |
| 7th anniversary | Indexation 7 | Indexed rent year 6 × index 7 |
| 8th anniversary | Indexation 8 | Indexed rent year 7 × index 8 |
| 9th anniversary = term | — | (New lease if renewed) |
Each indexation is calculated on the already indexed rent of the previous year, not on the initial rent. This cascade effect is crucial to correctly assess the theoretical rent at the end of the lease. 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 on 1 September 2017 at 800 € (average health indexes based on Statbel):
| Date | Health index August | Theoretical rent | Cumulative increase |
|---|---|---|---|
| Sept. 2017 (signature) | 105.84 | 800.00 € | — |
| Sept. 2018 (1st anniv.) | 107.71 | 814.12 € | +1.8 % |
| Sept. 2019 | 109.67 | 829.02 € | +3.6 % |
| Sept. 2020 | 110.98 | 838.90 € | +4.9 % |
| Sept. 2021 | 113.99 | 861.67 € | +7.7 % |
| Sept. 2022 | 122.82 | 928.35 € | +16.0 % |
| Sept. 2023 | 129.69 | 980.21 € | +22.5 % |
| Sept. 2024 | 132.42 | 1,000.84 € | +25.1 % |
| Sept. 2025 (8th anniv.) | 134.75 | 1,018.38 € | +27.3 % |
| Sept. 2026 (term) | — | (new lease) | — |
Over 9 years, the theoretical rent goes from 800 € to 1,018 € — i.e. +218 €/month cumulative. A single inflation shock (2022-2023) explains half of this increase.
To calculate your case, use the calculator by entering the signature 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 with continuation of the indexation chain).
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: 1st post-renewal indexation at the 10th anniversary date, then each year until the 12th
- The base rent for the chain remains the one resulting from the 8th indexation (renewal does not « reset »)
- 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 an automatic-written 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 | Automatic-written | 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 |
|---|---|
| Calculate each indexation on the initial rent (not in cascade) | Underestimates the rent due at end of lease (~5% below the theoretical over 9 years) |
| 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 |
| Index the 9th year (at term) | No indexation at term — it’s a new lease |
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 cascade automatically
- 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 lose 25% of purchasing power by end 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 cumulative annual indexations. The ninth year corresponds to the normal end of the lease — there is no further indexation possible if the lease is renewed, except for a new lease.
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 (automatic-written mechanism) 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: the first post-renewal indexation occurs at the next anniversary date, on the basis of the last indexed rent. The chain continues without interruption.
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 automatic 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. But the health index base differs (base 2013 for the 2018 lease, base 2025 for the 2025 lease), and the regional EPC coefficients do not apply the same way depending on the date (Brussels: coefficient for pre-14/10/2022 leases only). 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 (free procedure).