A separate and independent building sewer shall be provided for every building; except where 1 building stands at the rear of another on an interior lot and no private sewer is available or can be constructed to the rear building through an adjoining alley, court, yard or driveway, the building sewer from the front building may be extended to the rear building and the whole considered as 1 building sewer; provided, however, that apartment courts, motels and similar structures held under a single ownership shall be permitted to use a single sanitary sewer connection while the single ownership shall continue, the single connection to be a size and type approved by the Superintendent of Public Works.