What is the difference between usable square feet (what’s within my office) and rentable square feet (what’s referenced in my lease)?
Rentable square feet is the square footage that is used the calculate your rent. The difference between your rentable square feet and usable square feet has a few different terms — add-on factor, common area factor, and load factor.
Why the difference? The owner / landlord wants to earn income on every square foot of its building so divides the common areas among the tenants based on their percentage of use. The common areas all add value so the tenants are expected accordingly. Common areas can include elevators, stairwells, cafeterias, the lobby, and hallways.