The law regulates the transfer of rental apartments to third parties, especially subletting. This allows tenants, under certain circumstances, to let part of the rented apartment to third parties. 

The tenant must submit an application to the landlord for permission to sublet if he wishes to permanently let part of the apartment to third parties. This application should be made in writing. A legitimate interest in subletting must be presented. Legitimate interests can be financial reasons, for example if the tenant has less income due to a change in his job or wants to sublet part of his apartment because he is abroad. Personal reasons such as the desire for a roommate can also represent a legitimate interest. The landlord must give his consent to subletting if the tenant has a legitimate interest. Consent can only be refused for important reasons, for example if the apartment would be overcrowded or the potential subtenant is unreasonable for the landlord. Subletting creates a separate tenancy between the main tenant and the subtenant. The original tenancy between the landlord and the main tenant remains unaffected. Subletting is not permitted without the landlord's consent. Unauthorized subletting can be grounds for termination for the landlord. The permission to sublet can be revoked by the landlord under certain circumstances, for example if circumstances change and the tenant's legitimate interest no longer applies. 

The Ansbach Regional Court has commented on questions of evidence regarding unauthorized subletting. If a person who is not a tenant himself attaches his own nameplate to rented premises, this generally also documents joint ownership of the rental property. On one apartment there were nameplates belonging to people who had no contractual relationship with the landlord. The general view of the public understands that attaching a name to a mailbox means that mail that is posted in it reaches the person with the name. In addition, this suggests that the person with the name has (joint) ownership of the apartment. Ansbach Regional Court, decision of December 22nd, 2020, ref. 1 T 1379/20