The base of a structure is the starting point for all cumulative height measurements. It is defined as the sidewalk level at the ambient plain directly in front of the lowest exposed point where a structure's perimeter meets the ground.

Sidewalk level is taken instead of the actual entrance level, because in many cases there is a stairway or other elevation differential between the sidewalk and the entrance. When the entrance leads onto a sunken plaza or a man-made elevation such as a staircase, a line should be drawn perpendicular from the entrance to the sidewalk level before this height differential comes into play. The only exception would be if the lowest entrance leads out onto naturally sloping ground which continues further down to the sidewalk; in this case the entrance level itself would form the base.

The ambient plain may be elevated above grade level in special cases, such as highly developed urban districts where all surrounding streets are elevated above ground (e.g. at La Défense near Paris or Illinois Center in Chicago). Normally if a structure fronts on a body of water, the lowest surrounding built walkway is used as a reference point instead of water level (which may fluctuate too much to get a constant measurement).