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.