In an H Historical 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 100 feet and an area of not less than 12,000 square feet was in one single separate ownership on May 28, 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 date.