The Add-on factor is the difference between the usable area and the rentable area of an office building expressed as a percentage of the rentable area. This factor expresses the percentage of floor space for which a tenant pays rent but cannot derive benefit from due to columns, corridors, elevator shafts, stairwells, etc. within the area leased. Also called a 'load factor' or 'partial floor factor' or, when expressed as a percentage of the net usable area, an 'add-on factor'.