Fences, walks and hedges may be located in required yards as follows:
(a) If not at any point exceeding four feet in height above the elevation of the surface of the ground at such point, fences, walls and hedges may be located in any yard.
(b) If not at any point exceeding six feet in height above the elevation of the surface of the ground at such point, fences, walls and hedges may be located in any required rear yard or side yard, provided that on a corner lot, abutting in the rear the side lot line of another lot in an R District, no such fence, wall or hedge within twenty- five feet of the common lot line shall be closer to the side street lot line than the least depth of the front yard required on such other lot fronting the side street.