No driveway shall be constructed in a U-1 or U-2 District wider than twelve (12) feet. Any improved area behind a residential structure in a U-1 or U-2 District, that is improved with concrete, asphalt or similar hard surface material, shall not be utilized for the parking of motor vehicles, boats, or other large vehicles. All improved surfaces on a lot in a U-1 or U-2 District shall be included for purpose of calculating lot coverage.