On each lot not occupied by a building or structure and on each lot upon which a building is hereafter erected, enlarged or repaired, there shall be provided and maintained a lot area, yards and other open spaces which shall be not less than prescribed as follows.
A. Front yard depth: not less than 20 feet.
B. Side yard widths.
(1) Interior lot: not less than three feet on each side of the main building.
(2) Corner lot: not less than 20 feet on all yards abutting on a street or alley.
(3) Rear yard depth: not less than 20 feet. (Exclusive of any portion of depth to be used for permitted accessory uses or buildings.)