American National Standard Institute provides a manual on the way to measure the inside sq.footage of a home. The American National Standard For Single-Family Residential Buildings For Single-Family Residential Buildings
Download a copy for your office today.
Sometime you may come across differences between an appraiser’s measurement and what is on public record.
Assessor’s for public records may often just multiply the square footage of your first floor to get the second floor. Common discrepancies may occur in Cape-Cod 
style homes (or homes where the second floor doesn’t have the same footprint as the first. Lets say for example you have 800 sq.ft. on the first floor. During the assessment, an assessor may have simply multiplied 800 by 1.5 (1.5 representing 1.5 floors) to get 1,200 sq.ft. of living area.
Appraisers (different than the assessor) will measure according to ANSI (see below)
As you can see, an appraiser will not count any space below 5 feet in that area (assuming it meets the above criteria).
This is a great manual to have on hand. See how appraisers measure a home today.
