WOHNUNGSGEBERBESTÄTIGUNG
It is a declaration from Landlord to Residents registration office (Einwohnermeldeamt) that a tenant has moved in or out of their apartment. In order to register your Apartment, you need WOHNUNGSGEBERBESTÄTIGUNG form which should be duly signed by landlord (Vermieter). For a sublet contract (Untermietvertrag), existing tenant (Hauptmieter) has the signing authority.
