Building zone districts include all public or private streets, ponds, streams and waterways adjacent thereto, whether or not so designated upon the Building Zone Map, including lands under water and lands hereafter formed by filling, accretion, emergence, avulsion or otherwise. In case a public or private street, pond, stream or waterway shown on said map forms the boundary between two or more districts of the same class, the district shall be considered as continuous. In case a public or private street, pond, stream of waterway shown on said map forms the boundary between two or more districts of different classes, each district shall be deemed to extend to the center of the street or the center of the pond or the thread of the stream or waterway, as the case may be. Building zone boundaries shown upon the Building Zone Map, except where they follow streets or water courses, and except as provided in this subsection, are intended to follow existing property lines.