sloped ceiling half the room must be 7ft (except portions under obstructions which may be 6'4), and no less than 5 foot ceiling under the slopes
ANSI SAYS
"Ceiling Height Requirements
To be included in finished square footage calculations, finished areas must have a ceiling height of at least 7 feet (2.13 meters) except under beams, ducts, and other obstructions where the height may be 6 feet 4 inches (1.93 meters); under stairs where there is no specified height requirement; or where the ceiling is sloped. If a room’s ceiling is sloped, at least one-half of the finished square footage in that room must have a vertical ceiling height of at least 7 feet (2.13 meters); no portion of the finished area that has a height of less than 5 feet (1.52 meters) may be included in finished square footage
For a room to be included in the square footage calculation, the floor located under sloping ceilings must have a clearance of at least 5 feet (1.52 meters); further, at least one-half of the square footage in the room must have ceilings of at least 7 feet (2.13 meters) in height. For example, a one-and-one-half-story, 28 by 42 foot Cape Codstyle house has a first level with a ceiling height of 8 feet. On the second level, the ceiling has a maximum height of 9 feet but a minimum height of 4 feet at the walls as the ceiling slopes to match the pitch of the roof. All areas are finished. While the first level has 1,176 above-grade finished square feet, only that portion of the second level meeting the ceiling height requirements described above is included in the square footage calculation."