I just learned recently after I questioned a home appraisal that any space below grade, even fully finished living space, doesn't count towards the square footage of the home. Our home (my former home) in Idaho was 3208 sq ft. The appraisal listed it at half that and I questioned why. That's when I found out.

This explains it better than I can:

https://authorityappraisals.com/2019...s-living-area/