Here a small town or rural home with a walk out finished basement, the lower level will contribute less per SF largely because they tend to have mold, odors, etc. Sometimes they are little more than a large rec room, or maybe a bedroom and rec room. Those without walk out suffer even worse.
OTOH, lake properties where space is a huge premium and a 5,000 SF home (2 levels whatever) may be sited on a 12,000 SF lot. There, space is premium, and no one will be happy with an unfinished basement. That space will be equal to the upper level(s) and the primary living area is likely to be there. I am finishing up one with 8 rooms, 5 baths, indoor pool and a theater room. Since it isn't FNMA, I don't care about ANSI but the comps are all similar. All have boat docks (pers. prop) and are on small lots under one acre. It is utility that should determine GLA, not some trite handbook of nonsense like ANSI. The comparisons isn't some greater market. It is your comps. Are they similar construction?
So, a 2 story is generally NOT the same as a walk out basement. And I would be loath to use one as a comp for the other except where no real comparable exists, and that is rare.
BTW, I did this same house several years ago and realized I had made a huge mistake. After analyzing my land comps (vacant lakeside lots are scarce, I realized I probably should have valued the land solely upon the lake front footage. Previously I weighed the size of lot and not the frontage. But I also had few land sales. I had a couple of excellent sales to work with this time. I had some large (4 acre in one case) lot with only 75' of frontage and it sold for the same as a 15,000 SF lot nearby that had 120' of frontage. The surplus land was not as valuable.
