Hi Mark.... I am responding to your email. Here is how I handle this and how I teach my students.
If any portion of the improvement is below grade (below the threshold) then it, by fannie mae guideline, is to be reported as below grade even though there might be a walkout.
Some other ways to determine this are: 1. Is the floor slab? 2. Are the walls concrete?
Right now, IN MY MARKET, tract built average quality construction is around $88 SF above grade while below grade is closer to $38 SF for homes of median price, say $180,000. Please to not use these numbers since they apply only to MY MARKET. It can however indicate a possible ratio of above grade to below grade. Generally speaking, the below grade will have a low cost and also a lower market perception of value.
Do you have a Marshall/Swift residential cost book? Check out how it treats trilevel homes. Remember to adjust for your location and time. There is nothing wrong with using the same unit of adjustment for both above and below grade in the grid IF SUPPORTED BY THE LOCAL MARKET. Many appraisers use, say, $25 as an adjustment for size differences in GLA. This, obviously, does not represent cost...as in the cost approach. It is merely a number the appraiser determines to be appropriate for their market for adjustment. The same number could be used for below grade finished area.
If there is a walk-out, I prefer to adjust for this feature under functional utility rather than lumping it all together in the below grade box. Why? Because I can then handle two properties with below grade where one is a walk-out and the other isn't. You will notice two lines for below grade. The first line I use for raw square footage adjustments. The second line I use for finish. It might look something like this:
1000SF 95% Fin
3Rm,1Bd,1Ba
Of course, my way is not cast in bronze but I have found it easily illustrates how I make my adjustments and why.
Hope this is benefit to you, I wish you well!