There are strict requirements in the Civil Basic Code (BGB) that must be taken into account. Tenants are given time to consider whether or not to agree to the higher rent.
The only way for the landlord to increase the rent is to sue the district court for approval. The burden of proof is decisive, as reported by the Berlin Tenants' Association. The landlord must be able to prove that the rent increase is justified. If the landlord is right, the rent will be increased. If tenants and landlords do not agree on a rent increase, the court decides.