Fences, walls and retaining walls may be erected to a height of 48 inches only in the front yard, and to a height of 6 1/2 feet on any part of the zone lot not in the front yard, except that on corner lots no fence, wall, retaining wall or obstructing foliage shall be allowed within 20 feet of the point of intersection of the two property lines, or on curved property lines, the point of intersection of the property lines extended.