If you want to rent your apartment or one or more rooms, you have to sign a sublease form. This form must then be approved by the landlord. The owner can only refuse if:
the conditions of accommodation compared to the normal rental are abusive. This is the case when the sub-landlord demands a significantly higher rent than he pays himself. An additional charge of 10-20% is permitted for subletting furnished flats.
the owner is disadvantaged from the sublease: for example if you try to sublet a one room studio to three people. The form of sublease can not violate the principal lease form. The flat should for example not be subleased as a restaurant. The contracts must also be respected in cooperative flats.
the main tenant does not want to communicate the conditions of the sublease announce.