The rent may be increased by the proprietor, if inflation rises, the reference interest rate has been raised, the operating costs have actually increased or material alterations or renovations have been carried out. The landlord must tell you the rent increase by an official form. The form should tell you the reason for which rent increases.
The rent can be increased on the next termination date. The rent increase must be communicated at least 10 days before the beginning of the regular period of notice. If you do not agree with the increase, you need to tell the arbitration authority (not the one of your landlord) of the your residential district within one month. The letter must be signed by all residents who have signed the lease. For married couples, both spouses must sign. The letter should be sent (by registered mail) with a copy of the original lease, the previous rent change and the last increase.