The '''Add-on factor''' is the difference between the usable area and the rentable area of an office building expressed as a factor 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 [[column]]s, corridors, [[elevator shaft]]s, [[stairwell]]s, 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".
