A building may be erected on any lot which is not of the required minimum area or width and which was held in single and separate ownership as of June 8, 1964. In the case of such a lot which is 50 feet or more in width, the front and rear yard requirements of the district shall be observed, and the side yards may be reduced to two yards not less than 16 feet in aggregate width and neither less than six feet in width. In the case of a nonconforming corner lot, the side yard on the inside line may be reduced to not less than six feet and the side yard on the street side may be reduced to not less than 15 feet in R-A and R-1 Zones and to not less than 10 feet in R-2 Zones. In the case of such a nonconforming lot which is less than 50 feet in width, the plans for the proposed work shall be approved by the Planning Board acting as the Zoning Board of Adjustment to assure reasonable compliance with the spirit of the zoning regulations.