How much can rent increase and when is an increase unlawful?

The short answer: In Belgium, rent may be indexed annually according to the health index. Other increases are only possible after improvement works, through a market adjustment (after 3 years), or under a new contract.

Rent increases often create uncertainty for both tenants and landlords. What is allowed and what is not? When is an increase lawful and when can you challenge it? This article provides a complete overview of Belgian rules on rent adjustments.

Rent indexation: the legal framework

Annual indexation is the most common form of rent increase in Belgium. This adjustment follows the cost of living and is strictly regulated.

How does indexation work?

The formula is set out in law: New rent = base rent × new health index / base index

Example calculation:

• Base rent: €800 (January 2023)
• Base index: 120.45 (December 2022)
• New index: 125.67 (December 2024)
• New rent: €800 × 125.67 / 120.45 = €835

Important conditions:

• Indexation must be explicitly stated in the lease
• The landlord must notify this in writing
• Retroactive application is allowed for a maximum of 3 months
• The Statbel health index is authoritative

When may rent be indexed?

Indexation takes place on the anniversary of the contract. For a contract signed on 15 March 2023, the first indexation may only occur on 15 March 2024. The landlord uses the index for the month before the anniversary.

Note: Did the landlord forget to index? They may do so retroactively for a maximum of 3 months. After this period, the right to missed indexations lapses.

Other lawful increases

1. Increase after improvement works

If the landlord carries out substantial improvements, a rent increase may be justified. Examples include:

• New kitchen or bathroom
• Insulation works
• Installing central heating
• Extension of living area

Procedure:

1. Landlord proposes the works
2. Tenant must agree to the works and the rent increase
3. New arrangements are recorded in writing
4. Without agreement: no increase is possible

Practical tip: Always ask for a detailed quote and calculate whether the increase is proportionate to the investment.

2. Market-rate adjustment (9-year contracts)

For standard residential lease contracts of 9 years, there is a special option. After each 3-year period, the landlord may request an adjustment if:

• The current rent is significantly below market value
• They can prove this with comparable properties
• The justice of the peace agrees

Timeline:

• Years 0–3: no adjustment possible (except indexation)
• After year 3: first opportunity
• After year 6: second opportunity

Procedure via the justice of the peace:

1. Landlord summons with a market study
2. Comparison with at least 3 similar properties
3. Judge assesses reasonableness
4. Maximum increase: 20% of the existing rent

3. On contract renewal

A completely new contract means new negotiations. The landlord may then ask any price, but:

• The tenant is not obliged to accept
• Notice periods must be respected
• Discrimination in setting prices is prohibited

Recognising unlawful increases

Red flags:

1. Indexation without contractual basis
   • "We never indexed, so now 5 years at once"
   • Contract does not mention indexation
2. Incorrect calculation
   • Use of consumer price index instead of health index
   • Incorrect base index
   • Increase on top of already indexed rent
3. Unilateral increases
   • "Costs have risen, so the rent too"
   • "Other apartments in the building pay more"
   • "Energy prices have risen"

Frequently asked questions

May rent increase with new owners? No, a sale does not change anything about ongoing lease contracts. The new owner must respect all existing arrangements.

Can indexation be refused? If indexation is stated in the contract, this is a right of the landlord. You can, however, check the calculation.

What about energy renovations? Mandatory energy renovations (EPC standards) do not entitle the landlord to a rent increase. Voluntary improvements may, provided there is agreement.

Forgot to request indexation? The tenant does not need to spontaneously notify that indexation is possible. The initiative lies with the landlord.

Regional differences

Flanders

• Strict EPC standards from 2025
• No rent increase for mandatory renovations
• Rent premium possible for low incomes

Brussels

• Joint rent commission for disputes
• Reference rents per neighbourhood available
• Regional rent allowance exists

Wallonia

• Similar rules to Flanders
• Specific protection for student tenants
• Rent subsidies via AIS (Agence Immobilière Sociale)

Tips for tenants

1. Know your contract – Read especially the clause on price adjustments
2. Check annually – Use Statbel for index figures
3. Negotiate – For works: ask about added value vs. increase
4. Document – Keep all communication for 5 years
5. Be proactive – Report problems in time to avoid surprises

Tips for landlords

1. Be transparent – Explain indexation clearly
2. Respect deadlines – Maximum 3 months retroactive
3. Justify increases – Show the added value of investments
4. Follow procedures – Written communication avoids disputes
5. Stay reasonable – Extreme increases lead to vacancy

Conclusion

Rent increases in Belgium are strictly regulated. Annual indexation according to the health index is the most common and fully lawful if applied correctly. Other increases require either mutual agreement or intervention by the justice of the peace.

For tenants, it is essential to know the contract well and to scrutinise increases critically. For landlords, it pays to be transparent and follow legal procedures correctly. In case of doubt or disputes, tenants' unions, landlord organisations and the justice of the peace offer accessible support.

Belgian rental legislation seeks a balance between protecting tenants against arbitrary increases and ensuring a reasonable return for landlords. Knowledge of these rules prevents conflicts and supports a healthy rental relationship.