According to § 535 BGB, the landlord is obliged to leave and maintain the rental property in a contractually agreed condition. If he does not fulfill this obligation, the tenant is entitled to various rights under the law. In principle, it is possible to sue the landlord to have the defects rectified and thus repair the rental property. However, he can (additionally) also reduce the rent by an appropriate amount (rent reduction, § 536 BGB) and/or make use of his right of retention against the rent claim.