If no other method of increase is agreed in the lease agreement (e.g. annual indexation according to inflation), the landlord may unilaterally propose a rent increase up to the amount of the normal rent in the location, but not more than 20% over three years. The increase must be notified in writing and the tenant has three months to decide whether to agree to the proposal. Without a written proposal, the tenant does not have to agree to the higher payments.
If the parties do not agree, the landlord can apply to the court to have the court decide on the rent increase. A unilateral increase without any notice or agreement is therefore invalid and the tenant is not obliged to pay the increased rent