Here at the Greater Louisville Association of Realtors in Louisville, Kentucky and many other associations throughout the US, we use the American National Standards Institute (ANSI) rules on how to measure square footage. Until we adopted that standard, it was pretty much up to the individual Realtor to decide how to measure with many "customary" rules but nothing definite. Now we have certainty.
Examples of uncertainty clarified by ANSI (1) Do you count sq ft in the basement as sq ft of house? (2) Does it matter if part of the basement is above ground or if it is a walk-out? (3) What where the ceiling height is lower than the rest of the house? (4) What is the height it will be considered standard height and when will it be considered non-conforming height?
So non conforming sq ft will be those sq ft that are present in a home (example 300 sq ft of home where ceiling height is only 6') but do not conform to the ANSI standard (7' height required, with certain exceptions).
I hope this clarified the situation. Call any time with questions. 502-254-9600
Best regards, Neil Blumberg