When using English measurement units, the house is measured to the nearest inch or tenth of a foot; the final square footage is reported to the nearest whole square foot.
Rounding
The finished square footage of a house is to be reported to the nearest whole square foot for above-grade finished square footage and for below-grade finished square footage.
Openings to the Floor Below
Openings to the floor below cannot be included in the square footage calculation. However, the area of both stair treads 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.
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. 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.
Above- and Below-Grade Finished Areas
The above-grade finished square footage of a house is the sum of finished areas on levels that are entirely above grade. The below-grade finished square footage of a house is the sum of finished areas on levels that are wholly or partly below grade.