Industrial buildings are simple; the space is measured inside wall to inside wall. Multi-tenant office buildings, on the other hand, can be quite complex.
The Building Owners and Managers Association, (BOMA) provides measurement standards for commercial real estate.
For multi-tenant office buildings, three measurements apply.
1. Gross Square Footage: from the outside wall of the building to the opposite outside wall of the building. This is the SF used by the architect and contractor to price and build the building. It is also the SF used by the county to assess the building.
2. Rentable Square Footage. Floors are measured inside wall to inside wall, then any vertical penetrations of the floor, e.g. elevators, stairwells and ventilation shafts, are subtracted.
3. Useable Square Footage. The space is measured from the inside wall of common corridors to the midpoint of common walls between tenants to the dominant interior surface of the exterior wall.
The rentable area for each tenanted space is the useable are plus all the common areas of the building.