Commercial rents are almost always priced by the square foot -- that is, the amount of annual rent a tenant pays for each square foot of the space he uses. Raw warehouse or factory space might rent for only a few dollars per square foot; first-class office space or a desirable storefront might rent for $30 or $40 a square foot.

Square footage is a handy yardstick for comparing rents. But not all landlords measure square footage in the same way, so you need to ask two key questions: How much usable space will you get for your money, and how much of your rent is apportioned to "public" space such as lobbies, hallways, bathrooms, and mechanical areas (25% to 30% is typical)?

There are several standard formulas that real estate professionals use to measure and apportion space, particularly for modern high-rise office buildings. The "New York Multiple Tenancy Floor Method," for example, divides the cost of lobbies and bathrooms among the tenants on each floor, but excludes elevator shafts and stairways. The "BOMA International Method," on the other hand, allocates all the costs of common areas on multiple-tenant floors to the building as a whole, while the "Boston Method" charges tenants for the lobbies and hallways on each floor but not the bathrooms, elevator shafts, and mechanical rooms.Especially if you rent only a part of a floor, these differences can add up to large variations in rent over the lifetime of your lease, so it pays to know what you're getting.