Why does ANSI measure square footage from the outside of the walls?
Published August 27, 2026
How much wall thickness adds
A typical framed exterior wall is 6 to 8 inches thick including siding and drywall. On a 40 by 30 foot house, measuring outside instead of inside adds roughly 80 to 100 square feet, about 6 to 8%. Brick veneer or thick masonry walls add more.
This is one reason the number on an appraisal is larger than the sum of the rooms, and why a home measured with a laser inside each room comes out smaller than the same home measured to ANSI.
Upper floors and attached homes
You cannot walk around the outside of a second story, so it is measured from inside and the wall thickness is added back to reach the exterior dimension. Where the upper floor is the same footprint as the floor below, the appraiser uses the exterior dimensions from the ground floor.
For townhouses and other attached homes, the standard gives separate guidance for the shared wall, and the appraiser notes which convention was used.
What is not included
Exterior measurement does not mean everything inside the footprint counts. Garages, unfinished areas, and open-to-below spaces inside the exterior walls are still deducted. Those deductions follow the other rules in the ANSI Z765 guide.