Separating out what is guest house from dwelling GLA, if there are not floor plans on line, google earth you can look overhead at the roof structures and see the sep buildings.
Our public records online has overhead views that you can zoom in one, plus even a measuring tool (click on a corner and drag the mouse)
Front elevations driving by or online photos of MLS of the same model homes one can assume the breakout of guest house and living area is the same and check it online overhead.
Assessors include a guest house in living area (public records) because the purpose of assessors is to maximize the taxable base for the county or community. Thus, larger GLA is easier to tax than separating the structures out for their purposes.