Transition requirements. There shall be established, along the line of any side or rear lot that is contiguous to any residential district, unless the side or rear lot line coincides with any state or federal highway, a buffer area of at least 25 feet in width, plus five feet of additional width for each ten-foot interval or fraction thereof of the building height of the principal building. The buffer area shall be landscaped, and fencing shall be provided as required to provide appropriate screening of the lot adjoining the residential districts.