The minimum size of any front, side or rear yard shall be 100 feet for principal buildings and accessory uses and structures, except for permitted entry and exit drives and minor accessory structures and uses, such as necessary utility service facilities, bus stops and gatehouses, provided that such minor accessory structures and uses are shown on a site plan approved by the Planning Board. Except as permitted above, the one-hundred-foot minimum required yard setback area from street lines and property boundaries shall be maintained as a buffer area, either to be preserved in its natural state or to be landscaped in a manner to be approved by the Planning Board.