In a C Residence District, no building shall be erected on a lot having an area of less than 7,500 square feet and a width of less than 75 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 on July 25, 1954, having an area of less than 7,500 square feet and a width of less than 75 feet at the front street line; and provided, further, that in such case the lot width shall not be less than 60 feet and the area no less than 6,000 square feet.