Side Yard. None required for a building used exclusively for commercial or other nonresidence purposes, except that where a lot abuts a P Park-Conservation or Residence District a side yard shall be provided which shall be not less than 30 feet in width and where a lot abuts a street on the side lot line a side yard shall be provided which shall be not less than 20 feet in width. In any case, where side yards are provided, although they are not required, each such side yard shall be not less than 10 feet in width.