No accessory building or structure shall be located in any required front, side or rear yard. In cases where a front, side or rear yard is not required, and the accessory building or structure is voluntarily set back from an interior lot line, the accessory building or structure shall maintain an accessory yard of not less than five feet. In cases where a front, side or rear yard is not required, and the accessory building or structure is of wood frame or unprotected metal structure and is less than 500 square feet in floor area, the accessory building or structure shall maintain an accessory yard of not less than five feet from any interior lot line.