Any plot existing as a separate parcel in a residential district and not complying with the minimum lot, size, frontage or area on January 1, 1979, may, notwithstanding such fact, be improved with a building in accordance with all other regulations of this chapter, provided that the owner owns no adjacent land which may be used as a part of the plot, and provided further that, notwithstanding any provision above to the contrary, any plot existing as a separate parcel by virtue of a subdivision prior to January 1, 1979, may be improved with a building by compliance with all other regulations of this chapter and the requirements of the Planning Board.