A separate and independent building sewer shall be provided for every building except where one building stands at the rear of another on an interior lot and no 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 one building sewer; and except where spatial areas are insufficient to permit the installation of separate and independent building sewers.