It used to be (before last year) that Maryland state tax assessment does
not count rooms above garage, so I do not pay tax for 400+ sq ft.

MD still doesn't count any floor if there is any part of it below grade
(basement) or in the attic, even if it is finished with heating/air
conditioning.

However, MD count my neighbor's 2-level story rooms (foyer, family
rooms) as 2X of the area.

Those are in state assessment office (public record), and are used by
online apprasalers such as zillow and ditech.

However, the realtors use different methods to estimate my footage.

Then again, the apprasers use another method to count bedrooms and
living areas - I had 4 apprasals done a couple years ago when
refinancing. The two closest one are still 100 sq ft apart.