DEFINITION of 'Add-On Factor'
The number of usable square feet divided by the number of rentable square feet in a commercial real estate lease. The result of this calculation will be 1 if the two numbers are identical, but it is usually slightly lower than 1 because some square footage in a building will be partly or totally non-unusable. Non-usable square footage includes space shared with other tenants (such as lobbies, hallways, stairwells, elevators and restrooms) or occupied by structural components (such as support poles and interior walls). In a poorly designed building, the usable area may be considerably less than the rentable area.
BREAKING DOWN 'Add-On Factor'
In commercial real estate, the lease cost is calculated based on rentable area, which includes areas that are not usable. This means that for the same amount of usable space, a building with a lower add-on factor will cost the tenant less than a building with a higher add-on factor. The add-on factor is important because the tenant pays for this unusable space in his lease. He pays for a fraction of shared common areas and he pays for all of the space he rents that is occupied by structural components. Potential tenants can thus use the add-on factor to help them compare leases and determine which lease offers the best value.