A nonconforming dwelling in any R or A district, where the nonconformity consists only of deficiency in yard dimensions or the required parking spaces and where no such deficiency exceeds fifty (50) percent of the requirements of the district, or any dwelling in an A district which is located on a building site of at least five acres and said parcel was of record prior to May 5, 1972, may be structurally altered or enlarged; provided, that any addition or enlargement shall itself be fully conforming and that the number of dwelling units therein shall not be increased. (Prior gen. code § 8-62.4)