Any parcel of land with an area or width less than that prescribed for a lot in the R-A-1 (one family); R-A-2 (one family); R-B (one family); R-B (two family); R-C (one/two family) in which such lot is located, which parcel was under one ownership at the date of the adoption of this ordinance, when the owner thereof owns no adjoin land, may be used as a lot for the construction of a one-family as may be permitted in the residential district wherein such lot is situated provided, however, that no existing non-conforming lot may be subdivided into two or more lots.