Required open space and exceptions. All required yard and other open areas shall be maintained with no portion of such area used as building area or off-street parking or driveways. The following shall constitute the only exceptions: permitted animal feeding areas, arbors and trellises, barbecues or outdoor fireplaces, building projections, cultivated fields or gardens, fences, flagpoles and playground equipment or games, ponds and streams, sewage disposal systems, swimming pools, temporary garden structures, tree wells, water systems, air conditioning units or private power generating units.