Rent indexation in Belgium: how to calculate it with the health index

What is rent indexation and why the health index?

Indexation adjusts the rent to the rising cost of living. It is not a rent increase in the strict sense: you simply keep the original rent in line with price developments. The Flemish Residential Lease Decree (Vlaams Woninghuurdecreet) requires you to use the health index (gezondheidsindex). This is the consumer price index excluding alcoholic beverages, tobacco and motor fuels. Statbel, the Belgian statistical office that is part of FPS Economy, calculates and publishes it every month.

You may not use any other index, even if the contract says so. A clause allowing more than the statutory indexation is reduced to the legal maximum. This guide follows the Flemish rules. Since the sixth state reform, Brussels and Wallonia have their own lease legislation, with different details on the starting index and their own energy measures.

The rent indexation formula step by step

The formula is set out in article 34 of the Flemish Residential Lease Decree: indexed rent = base rent × new index ÷ starting index. Each element has a precise meaning:

• Base rent: the rent agreed in the contract (or after any rent revision), excluding costs and charges.
• New index: the health index of the month before the anniversary of the date the lease took effect.
• Starting index: the health index of the month before the month in which the lease (or the rent revision) took effect.
• Use the same base year for both figures. Statbel publishes the health index in several base years (including 2013 = 100 and 2025 = 100). Mixing them gives a wrong result.
• Costs and charges are not indexed with this formula. A fixed lump sum for costs stays fixed unless you both agree otherwise.

Which starting index do you use?

For leases concluded in Flanders after 31 December 2018, the starting index is the health index of the month before the month in which the lease took effect. Note that this is the start date of the tenancy, not the date you signed. If the tenancy starts on 1 September, the starting index is the one for August.

Older contracts may follow a different rule, because the federal lease law applied at the time. If you are unsure about a contract from before 2019, use the Statbel rent calculator: it asks for the start date and applies the correct regime.

The new index is the health index of the month before the anniversary. Statbel publishes a month's index at the end of that month. For an anniversary on 1 September, you therefore only know the August index at the end of August. Keep this in mind if you want to inform your tenants on time.

Worked example with real index figures

Suppose you rent out a studio with a base rent of €450 per month. The lease took effect on 1 September 2025. You want to index on 1 September 2026.

• Starting index: health index August 2025 = 135.64 (base 2013 = 100).
• New index: health index August 2026 = 140.30 (base 2013 = 100).
• Calculation: 450 × 140.30 ÷ 135.64 = €465.46.
• The rent therefore rises by €15.46 per month, about 3.4 percent.
• If you only request the indexation in writing on 1 February 2027, you can claim the higher rent back to 1 November 2026 at the earliest. The difference for September and October is lost.

When and how may you index the rent?

Indexation is a right of the landlord, but it does not happen automatically. For a main residence lease in Flanders, these conditions apply:

• The lease is concluded in writing. With an oral lease, indexation is not possible.
• Indexation is not expressly excluded in the contract.
• You index at most once per rental year, on the anniversary of the date the lease took effect.
• You request it in writing from the tenant: a letter or email with the calculation (base rent, both index figures, new rent and effective date) is sufficient. Keep proof that you sent it.
• The indexation applies retroactively for at most three months before the month of your request. If you forget to index, you lose money.
• You do not need to index every year to keep your right. Because the formula always uses the starting index, a later indexation can bring the rent straight to today's level. Only past amounts you missed cannot be recovered, apart from those three months.

EPC correction: does the 2022-2023 energy measure still apply in 2026?

During the energy crisis, Flanders limited indexation for less energy efficient homes. Between 1 October 2022 and 30 September 2023: with EPC label D you could pass on only half of the normal indexation, and with label E or F or no EPC you could not index at all. Homes with label A+, A, B or C could be indexed normally.

That period is over, but the measure still has an effect, also in 2026. Since 1 October 2023 you multiply the result of the normal formula by a correction factor, so that the frozen indexation is not caught up. This applies to main residence leases that took effect before 1 October 2022 and that had label D, E or F or no EPC during the crisis period. For label E, F or no EPC, the factor is the ratio between the 2022 and 2023 health index of the relevant months; for label D, half of the difference is corrected.

If you obtain a new EPC with label A+, A, B or C after 1 October 2023, you may index fully again from that moment, without a correction factor. This cannot be applied retroactively. The correction factors do not apply to student leases, nor to leases that took effect on or after 1 October 2022.

Indexation of student leases

Student rooms fall under title III of the Flemish Residential Lease Decree, with specific rules in article 61. A student lease often runs for ten or twelve months, and then there is nothing to index during the contract: the annual adjustment on the anniversary only applies if the lease runs longer than one year, unless it is expressly excluded.

More important is what happens when the same student stays in the same room for the next academic year. The base rent of the new contract may then not exceed the rent of the first contract, indexed with the formula above. You can only ask more if the normal rental value has increased by at least 20 percent because of new circumstances, or by at least 10 percent because of works in the room. Two consecutive ten-month contracts with a summer break in between also count as consecutive.

• If you sign the new contract in spring, the new index is not yet known. State in the contract that the indexed base rent applies, and tell the student the amount once the index is published. Otherwise you risk being deemed to have waived indexation.
• A lump sum for costs may not be indexed. In a new contract with the same student you may adjust the lump sum by mutual agreement.
• This limit does not apply to a new tenant: you then agree a new rent.
• Article 61 does not apply when an educational institution rents to eligible students at a reduced price.

How Kotmaster helps with rent indexation

Indexation is simple arithmetic, but with ten or fifty rooms it is easy to miss an anniversary or a written request. Kotmaster indexes rent automatically based on the health index, so you do not have to look up index figures yourself. The new amounts feed into the automatic payment follow-up with reminders. You create contracts from templates and have them signed digitally with itsme®, and everything is stored centrally. Also read our guides on the student room rental contract and registering your lease via MyRent, or get in touch for a demo.

This guide provides general information on the Flemish rules and is not legal or tax advice. If in doubt, check the current legislation or consult a specialist.