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Do Stairs Count as Square Footage?

Short answer: yes. Under ANSI Z765, stairs count as square footage on the floor from which they descend, which is the upper floor in a two-story home. The stair opening on that floor is not deducted as open to below, and the lower floor is measured normally to its exterior walls.

The rule: stairs belong to the floor they descend from

Under ANSI Z765 and standard appraisal practice, gross living area is measured from the exterior perimeter on each floor. For a two-story home:

Second floor: ANSI Z765 (2021) includes stairways in the square footage of the floor from which they descend. Trace the second floor's full exterior perimeter and leave the stair opening in. It is not an open-to-below void and is not deducted.
First floor: Measured normally, from exterior wall to exterior wall. You don't add or remove anything for the stairs.

The staircase connects the two floors, and the standard assigns it to the upper one. So the opening the stairs pass through never reduces the second-floor number.

How to handle this in MeasureFloorPlan

When tracing a multi-story home in MeasureFloorPlan, handle the stairs in two steps:

Trace the full second-floor perimeter: Draw one polygon around the outside walls of the second floor. Don't add a cutout for the stair opening, because the stairway counts as part of this floor.
Trace the first floor normally: Draw its exterior perimeter as usual. Don't route the shape around the base of the stairs.

That matches ANSI Z765 and keeps the sketch easy to audit. Save the cutout tool for real open-to-below areas like a two-story foyer.

What about open-to-below areas?

A stairwell is not treated as open to below. Real open-to-below areas are different: in a two-story foyer or a great room that rises through both levels, the upper level has no floor, so that area is left out of the upper floor's GLA. The lower floor under the opening is counted normally.

Common mistakes

Deducting the stair opening: Treating the stairwell as an open-to-below void and subtracting it from the second floor. This understates GLA by the stairwell area (typically 40 to 80 sq ft in residential homes).
Carving the stairs out of the first floor: Routing the first-floor trace around the base of the stairs. The first floor is measured to its exterior walls like any other level.
Ignoring landings: Landings are finished floor space at an intermediate level. They count as GLA if they are above grade, finished, and heated, and the ceiling height at the landing meets the ANSI Z765 minimum of 5 feet.

Does stair square footage matter much?

In most homes, a stairwell is 40 to 80 square feet. At $100 per square foot GLA adjustment (a typical mid-market price per square foot rate), that is a $4,000 to $8,000 difference in appraised value if counted wrong. It is not a trivial error when the staircase runs multiple flights or when the home is in a high-cost market.