Rentable square footage of a given space includes factoring in a proportionate percent of common areas such as elevator shafts, common corridors, stairwells. Useable square footage is what you are left with. If thirty per cent of the building consists of common area, then the loss factor should translate into about 30%. As an example, a rental property listed as one thousand RSF (Rentable Square Feet) should yield approximately seven hundred USF (Useable Square Feet).