For corner lots platted after March 20, 1969, the street side yard shall be equal in width to the setback regulation of the lots to the rear having frontage on the intersecting street. On corner lots platted and of record on March 20, 1969, the side yard regulation shall apply to the longer street side of the lot, except in the case of reverse frontage where the corner lot faces an intersecting street. In this case, there shall be a side yard on the longer street side of the corner lot of not less than 50 percent of the setback required on the lots to the rear of such corner lot, and no accessory building on such corner lot shall project beyond the setback line of the lots in the rear; provided further, this regulation shall not be so interpreted as to reduce the buildable width of the corner lot facing an intersecting street and of record, or as shown by existing contract of purchase on March 20, 1969, to less than 28 feet or to prohibit the erection of an accessory building.