Total Rentable Area is calculated by adding the occupant area to a prorated portion of the shared building common spaces like shared corridors, restrooms and elevator lobbies.
The red area at the left would represent rentable area.
The rentable area excludes major vertical penetrations, such as stairwells, elevators, and major shaft spaces, which are not shown above.
Calculate based on RSF (rentable) = USF (useable) + prorated share of Building Common Areas.
See ANSI/BOMA Z65.1* for detailed calculation method. Generally speaking, add the usable area to building common spaces while excluding major vertical penetrations, like an elevator shaft, or stairwells.