In a B Residence District, no building shall be erected on a lot having an area of less than 10,000 square feet and a width of less than 80 feet at the front street line; provided, however, that a single-family dwelling may be built upon a lot held in single and separate ownership at the effective date of this amendment of this chapter, viz., July 25, 1954, having an area of less than 10,000 square feet and a width of less than 80 feet at the front street line; and provided, further, that in such case the width of the lot shall not be less than 60 feet and the area not less than 6,000 square feet.