In most major commercial office markets, leasing is typically done on a monthly or annual “rent per square foot” basis. So, in order to calculate the total rent all you need to do is multiply the rental rate by the unit size, right? Not so fast. Unit size can change like the weather and tenants are well advised to negotiate not only the rental rate, but the unit size as well.

First, the tenant needs to recognize that there are different types of area measurements. Gross floor area refers to the size of the building envelope, from outside wall to outside wall. Useable area, sometimes known as carpetable area refers to the portion of the floorplate that will actually be used by the tenant. And, rentable area (which, as the name implies, is what the tenant’s rent is based on) refers to the size of the actual tenant suite, including all mechanical rooms, stairwells and elevator shafts, plus an add-on for common areas (for example, a pro-rate portion of the corridor for a multi-tenant floor, and the lobby). The spread between the tenant’s actual useable area and the rentable area can be as much as 30% or more, depending on the efficiency of the building.

Unless the tenant is going to retain his own architect to measure the spacce (unlikely for all the but the largest space users), the rentable area is, simply, whatever the landlord says it is. We saw this first-hand when leasing office space last year. For some strange reason, most of the 5,000 square foot suites we looked at measured 50′ x 90′.

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