In a BB Residence District, each lot shall have a frontage of at least 90 feet, a depth of at least 100 feet and an area of at least 12,500 square feet, except that, if a plot having a frontage of not less than 50 feet was in one single separate ownership on September 10, 1945, or if a plot having a frontage of not less than 60 feet was in such ownership on January 6, 1947, or if a plot having a frontage of not less than 70 feet was in such ownership on the 26th day of July 1954 or if a plot having a frontage of not less than 90 feet and an area of not less than 10,000 square feet was in such ownership on the 28th day of May 1987, a building may be erected on such plot, provided that all other provisions of this Code are complied with, and further provided that such single and separate ownership is continuously maintained following the aforesaid dates.