Where two or more lots, created by the filing of a map pursuant to the Map Filing Law prior to establishment of the Planning Board, have any contiguous lines and are in single ownership and one or more of the lots is nonconforming in any aspect, the lots involved shall be considered to be an undivided parcel for the purposes of this chapter and no portion of said parcel shall be conveyed or divided except through the filing of an approved subdivision in accordance with the provisions of this chapter.