A principal tenant always concludes a tenancy agreement with the owner of the apartment or leaseholder/tenant of the entire building. If the principal tenant lets parts or the entire apartment to a third party, this is considered to be a sublease. Generally, it is not permitted to sublet the apartment unless it takes place with the knowledge of the apartment owner or if only parts or individual rooms are sublet.  If the tenancy between the principal tenant and the landlord is terminated, the sublease agreement is also terminated.