Grossing factor
term used in the real estate and commercial construction sector to describe the difference in the rentable square footage to the useable square footage.
Grossing factor relates the Rentable square footage, which includes common area space such as hallways, common bathrooms and possibly lobbies; to the useable square footage, which includes only that space which you would specifically occupy for your buisness needs, ie - the area you can carpet. The grossing factor is usually listed as a %.