As a landlord, you have the right to adjust the rent annually to keep up with the cost of living. You do this through rent indexation — a calculation based on the official health index published by Statbel. It sounds technical, but it is simpler than it seems. In this article we explain step by step how it works.

What exactly is rent indexation?

Rent indexation is the landlord's right to adjust the base rent annually in line with the evolution of the health index.1 This is not a free price increase: the new rent is strictly calculated using a legal formula.

The health index is an official figure published monthly by Statbel (the Belgian statistical office). It measures the price evolution of a basket of goods and services — excluding tobacco, petrol and diesel.

📌 Important Indexation is a right, not automatic. You must request it yourself via a written notification to the tenant. Without this letter, the indexation does not take effect.

The calculation formula

The legal formula for rent indexation is the same in every region:2

Legal formula (art. 1728bis Civil Code)
New rent = Base rent × New health index ÷ Base index
Example: Base rent €800 · Base index 133.73 · New index 137.55
→ €800 × 137.55 ÷ 133.73 = €822.87

What is the base rent?

The base rent is the rent as stated in the lease agreement — not the indexed rent from the previous year. If you have indexed before, you always calculate from the original contract rent.

What is the base index?

The base index is the health index for the month before the lease was signed.3 If the contract was signed in February 2022, for example, you use the index for January 2022. You can find this value on the Statbel website, or our calculator and rent tool retrieves it automatically.

What is the new index?

The new index is the health index for the month before the contract anniversary. If you are indexing on 1 March 2026, you use the index for February 2026.

Step by step: how do you proceed?

  1. Check whether you are allowed to index
    You may only index after the first year of the contract. Is the contract less than 12 months old? Then wait for the first anniversary. Have you indexed before? At least 12 months must have passed since the last indexation.
  2. Look up the base index
    Go to statbel.fgov.be or use our calculator. Enter the month before the lease start date.
  3. Look up the new index
    The health index for the month before the contract anniversary. Statbel publishes this every month.
  4. Apply the formula
    Calculate the new rent: Base rent × New index ÷ Base index. Round to 2 decimal places.
  5. Send a written notification
    Send the tenant a letter or email with the calculation. The indexation only takes effect from the month after notification — or up to 3 months retroactively if you are late.

What if you forgot to index?

No need to panic. You may apply indexation retroactively, but with a limitation: maximum 3 months in the past.4 If you are more than 3 months late, that increase is permanently lost for that period.

⚠️ Watch out if you have not indexed for a long time Suppose you have not indexed for 2 years. You can still index today, but only using the index from the anniversary date that falls within the past 3 months. Concretely: of the 24 months you have not indexed, 21 are permanently lost. Only the last 3 months can still be recovered.

Watch out: EPC rules since 2023

Since 2023, stricter rules apply in Flanders for properties with a poor energy performance certificate (EPC).5 For contracts concluded from 1 January 2023:

  • EPC label A, B or C: full indexation permitted
  • EPC label D: indexation limited to 50% of the increase (2026–2027)
  • EPC label E or F: indexation temporarily not permitted (2026–2030)
  • No EPC or G: indexation completely prohibited

Similar but slightly different rules apply in Brussels and Wallonia. Our calculator and rent tool automatically takes into account your region and EPC label.

Calculate your rent indexation in 30 seconds

Enter your contract date and rent — the calculator does the rest automatically. Including the correct health index and EPC correction.

Go to calculator →

The indexation letter: what must it contain?

The law requires a written notification, but does not prescribe a mandatory format. Nevertheless, there are elements you should always include:6

  • The base rent and the year of the lease
  • The base index used (month + value)
  • The new health index (month + value)
  • The calculation with formula
  • The new rent and the effective date
  • A reference to Article 1728bis of the Civil Code

A correct letter protects you as a landlord and leaves no room for dispute. Our rent tool automatically generates a compliant letter based on your details.

Legal sources

  1. Article 1728bis of the Civil Code (Law of 20 February 1991 containing special provisions regarding lease agreements for primary residences).
  2. The same formula applies in Flanders, Brussels and Wallonia; rental legislation is regionalised but the indexation formula remains anchored at federal level.
  3. Statbel publishes health index figures monthly at statbel.fgov.be. The base index is the index for the month before the contract was signed (not the start date).
  4. The 3-month retroactive limit follows from case law and the prevailing interpretation of Art. 1728bis §3 of the Civil Code.
  5. Decree of the Flemish Government of 2 September 2022 introducing the energy correction for rent indexation; applicable to lease agreements concluded from 1 January 2023. See also the Flemish Residential Tenancy Decree.
  6. Although no strict formalities are required, it is recommended to send the letter by registered post or by email with read receipt.