Where a district boundary line established in this chapter or as shown on the Zoning Map divides a lot which was in single ownership as of the time of the passage of this Ordinance, authorized thereon and the other district requirements applying to the least restricted portion of such lot under this Zoning Ordinance shall be considered as extending to the entire lot, provided the more restricted portion of such lot is entirely within twenty-five feet of the dividing district boundary line. The use so extended shall be deemed to be conforming.