We used to be able to treat these as all GLA. Was a lot easier, because that is the way buyers see them. There is no reason this has to be so complicated, except FNMA forms told us so. We used to be able to get exceptions in the SouthWest, but since everything has to be in a Box now, we have to do it the same as those in other parts of the country. Splits are almost impossible to appraise this way, because no one knows the actual "Basement" sf on comps. MLS and county count everything as GLA. FNMA, FHA and VA have one solution to this, they all told me to my face (At the Valuation Expo) to "Guess" the sf of the basement on split level and basements on the comps.
So guessing is more reliable than just including the lower level in GLA.
I am so happy that in a few years I will be retiring.
P.S. I have talked to a local FNMA reviewer about this and he said that you can include finished basements and the lower level of a split level in the GLA on your appraisal, as long as the subject and comps are counted consistent. The only problem is that FNMA will not admit this, and AMC and lenders do not know this.