In an A Residence District, each lot shall have a frontage of at least 100 feet, a depth of at least 100 feet and an area of at least 15,000 square feet, except that, if a plot having a frontage of not less than 50 feet was in one single and separate ownership on September 10, 1945, or if a plot having a frontage of not less than 60 feet was in such ownership on May 4, 1946, or if a plot having a frontage of not less than 70 feet was in such ownership on the 26th day of June 1954, or if a plot having a frontage of not less than 100 feet and an area of not less than 12,000 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.