Space which has been counted or calculated as part of a side yard, rear yard, front yard, court, lot area, or other open space to meet the requirements of this ordinance for a building, shall not be counted or calculated to satisfy or comply with a yard, court, lot area, or other open space requirement for any other building. An open porch or paved terrace may occupy a required front yard or rear yard, provided that the unoccupied portion of the front yard or rear yard furnishes a depth of not less than 21 feet.