Fence, walls or hedges in residential zones may be erected, altered or reconstructed to a height not exceeding six feet above ground level when located to the rear of the front building line and not exceeding three feet in height when located in front of the front building line or in a principal front yard. Any walls, fences or hedges erected, altered or reconstructed in a secondary front yard shall be no closer than 15 feet to the curbline, and in no case closer than three feet to the property line.