The landlord may pass on cost increases to the tenant and increase the rent accordingly (Art. 269a lit. b CO). If the increase is not justified or is too high, this results in an unfair rent. If costs decrease, the tenants can claim that the rent has become unfair due to the decreased costs and demand a rent reduction.
The change in costs since the last rent determination is decisive.
In particular, increases in the mortgage interest rate, fees and property taxes, building lease interest, insurance premiums and increases in maintenance costs are considered cost increases (art. 12 VMWG).
The most important criterion is the mortgage interest rate or the reference interest rate (Art. 12a VMWG). An increase in the mortgage interest rate or the reference interest rate by a quarter of a percent entitles the tenant to increase the rent. The formula for calculating rent reductions is not specified in the ordinance.
| Mortgage rate | Increase rate | Reduction rate |
| Lower than 5% | 3 Percent | 2.91 Percent |
| Between 5 and 6% | 2.5 Percent | 2.44 Percent |
| Higher than 6% | 2 Percent | 1.96 Percent |
In the case of further changes in costs (increase or decrease), the party claiming the change must prove it. In practice, the conciliation authorities use lump sums for costs.
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