I got this via a google search, that took me back to the forum here"
http://appraisersforum.com/ask-appraiser/69993-correct-definition-above-grade-gross.html
Per the Fannie Mae selling guide dated 4/12/02. Section 405.06.
Section 405.06 – Gross Living Area
The most common comparison for one-family properties (including units in PUD, condominium, or
cooperative projects) is above-grade gross living area. The appraiser must be consistent when he or she
calculates and reports the finished above-grade room count and the square feet of gross living area that is
above-grade. For units in condominium or cooperative projects, the appraiser should use interior
perimeter unit dimensions to calculate the gross living area. In all other instances, the appraiser should
use the exterior building dimensions per floor to calculate the above-grade gross living area of a property.
Only finished above-grade areas should be used--garages and basements (including those that are
partially above-grade) should not be included. We consider a level to be below-grade if any portion of it is
below-grade-regardless of the quality of its "finish" or the window area of any room. Therefore, a walk-out
basement with finished rooms would not be included in the above-grade room count.
Rooms that are not included in the above-grade room count may add substantially to the value of a
property-particularly when the quality of the "finish" is high. For that reason, the appraiser should report
the basement or other partially below-grade areas separately and make appropriate adjustments for them
on the "basement and finished areas below-grade" line in the "sales comparison analysis" grid. To assure
consistency in the sales comparison analysis, the appraiser generally should compare above-grade areas
to above-grade areas and below-grade areas to below-grade areas. The appraiser may deviate from this
approach if the style of the subject property or any of the comparables does not lend itself to such
comparisons. However, in such instances, he or she must explain the reason for the deviation and clearly
describe the comparisons that were made.