For corner lots, platted after the effective date of these regulations, the street side yard shall be equal in width to the minimum required side yard for the district in which it is located, plus 20 feet; i.e., for a minimum required side yard of 10 feet the street side yard shall be not less than 30 feet; provided, however, this regulation shall not be interpreted as to require a side street yard of greater width than the minimum required front yard width. For corner lots, platted after the effective date of these regulations, the minimum required lot width shall be increased by any amount not less than 20 feet so as to allow for the additional required street side yard, i.e., for a minimum required lot width of 80 feet, the minimum width of a corner lot shall be not less than 100 feet. On corner lots platted and of record at the time of the effective date of these regulations, the same regulations shall apply except that this regulation shall not be so interpreted as to reduce the buildable width of the corner lot of record or as shown by existing contract of purchase at the time of the effective date of these regulations, to less than 28 feet or to prohibit the erection of an accessory building.