The drainage system of each new building and of new work installed in existing buildings shall be separate and independent from that of any other building and, when available, shall have an independent connection with a public sewer. Provided, however, when one building stands in the rear of another building on an interior lot and these buildings are on one parcel of land under one owner, then the building sewer may be extended to serve the rear building.
(`87 Code, § 32-53)