Eighteen (18) feet above the underside of the actual first floor, measured at the side wall, or twenty (20) feet, six (6) inches above the grade of the side wall, whichever is lower.
As used in this section, an "addition to an existing structure" does not include construction over a new foundation or construction cantilevered beyond a new or existing foundation, "grade of the side wall" means the average grade of the highest and lowest points of the natural grade of the portion of the lot covered by the side wall, and "grade of the side setback line" means the average grade of the highest and lowest points of the natural grade of the portion of the lot directly below the side setback line. No portion of the structure shall intrude beyond the daylight plane except for dormers and gables as provided in subsection (2) of this section and chimneys, vents, flues and solar collectors.