Structures. All structures, whether attached to the principal structure or not and whether open or enclosed, including porches, balconies or platforms above normal grade level, shall not project into any minimum front, side, or rear yard, except that structures accessory to single-family residences may extend partly into required rear and side yards, provided that required side and rear yards are not diminished by more than five feet and that no side or rear yard shall be less than five feet. Additionally, not more than one structure which is not more than 100 square feet in size may be located in any required rear or side yard in any residential district.