Every habitable building hereafter erected shall be located on a zoning lot and in no case shall there be more than one such building on one zoning lot, except that in a two-story garage with living quarters upon the second floor, such quarters may be occupied by a servant (and his or her family) of the family occupying the main structure. There may also be constructed a guest house (without a kitchen) or rooms for guests within an accessory building, provided that such facilities are used for the occasional housing of guests of the occupants of the main structure and not for permanent occupancy by others as a housekeeping unit.