Around here, the tax assessment counts finished space. So a finished attic/basement would count, but unfinished wouldn't.
In order to get around the taxes, many people will "nearly finish a basement by doing things like leaving off trim and not gluing down carpet/flooring. or go the other direction, and put down proper flooring/trim, but leaving the ceiling open and spraying paint over everything to create an industrial sort of feel.
The net effect of this is that you take SFA with a grain of sale and expect it to be optimistic. From a buyer's perspective useable space, flow and layout are more important than Total SF anyways. I've lived in very big, small houses and in very small, big houses.