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.<ref>{{cite web|title=Terms: Add-on Factor|url=http://lease.io/terms/add-on-factor|publisher=LeaseMatrix|accessdate=13 May 2013}}</ref> Also called a "load factor" or "partial floor factor" or, when expressed as a percentage of the net usable area, an "add-on factor".
