We follow the ANSI Z765-2021 standard.
Based on this standard, the area of the stairs is included in the level from which they descend. The area below the stairs is included in the level on which they land - regardless of finish or ceiling height.
Please see the information below taken from the ANSI documentation:
“A common construction practice is to provide a floor opening for stairs that is the same size as the stairs themselves. Therefore, the area of stairs included in finished square footage is typically equal to the area of the opening in the floor. For example, a two-story, 28 x 42 ft. house embodies 1,176 finished square feet on the first level and 1,176 finished square feet on the second level, provided that all areas are finished and the opening in the floor of the second level does not exceed the area of the stair treads. Further, stairs that descend to an unfinished basement are included in the finished square footage of the first level regardless of the degree of finish of the stairs or the degree of finish of the area around the stairs. Finished stairs suitable for year round use ascending to an unfinished upper area are included in the square footage calculation of the floor from which they descend. In addition, areas beneath stairs are included in the finished square footage regardless of the distance between the stairs and the floor below or of the degree of finish of that area.”
3.4 Openings to the Floor Below:
“Openings to the floor below cannot be included in the square footage calculation. However, the area of both stair reads and landings proceeding to the floor below is included in the finished area of the floor from which the stairs descend, not to exceed the area of the opening in the floor.”
3.6 Ceiling Height Requirements:
“To be included in finished square footage calculations, finished areas must have a ceiling height of at least 7 ft. (2.13 m) except under beams, ducts, and other obstructions where the height may be 6 ft. 4 in. (1.93 m); under stairs where there is no specified height requirement; or where the ceiling is sloped.”