Where a zoning district boundary line divides a lot other than by following a stream or street, any use permitted in either district may be extended not more than 20 feet into the adjacent district. A use permitted in the zoning district so extended shall thereafter be a permitted use in the extended area. A zoning district line, however, shall be altered only once by utilizing this subsection of the chapter, after which the lot use shall be governed by the regulations of the zoning district in which it is located after the zoning district boundary line adjustment.