In R districts, single-family dwellings only may be erected on any parcel of land, the area of which is less than the building site area required for the particular district in which the parcel is located, but if and only if the parcel was in single ownership at the time of the adoption of the ordinance codified in this title. No structure shall be erected on any substandard parcel if the parcel was acquired from the owner or owners of record of contiguous property or the contiguous owner's or owners' transferee after the time of adoption of the ordinance codified in this title. (Ord. 3-83 § 5.18, 1983)