A rent index gives you an overview of the rents of comparable apartments in the city or municipality. It contains indications as to

• whether a landlord's request for a rent increase is justified,
• whether the rent is permissible for re-letting if the apartment is located in an area in which the provisions of the so-called rent cap apply (Annex to Section 1 of the Tenant Protection Ordinance),
• whether the rent is generally reasonable (outside the scope of the so-called rent freeze).

The rent index shows the average net cold rent (local comparative rent) in euros per square meter per month on the basis of net cold rents that have been newly agreed or changed in the last six years. This comparative rent can be higher or lower for each individual apartment. The criteria for this are listed in the rent index.

Decisive factors can be, for example

• Apartment size
• Age of building
• residential location
• Energy modernization status and
• the furnishings of the apartment, e.g. central heating, built-in furniture, parquet flooring.

The local comparative rent may differ from the current market rent.

Rent indexes are drawn up by the cities/municipalities and by representatives of landlords' and tenants' interests, for example tenants' associations, homeowners' and landowners' associations. A city/municipality should draw up a rent index if there is a need for one.

Since July 1, 2022, municipalities with more than 50,000 inhabitants have been obliged to prepare rent indexes.

As a rule, rent indices are to be adjusted to market developments every two years.

There are rent indexes in many larger cities and municipalities.