Guide
How Stairs Are Counted in Square Footage
How ANSI Z765 counts finished stairs and landings, which floor they belong to, and the common mistakes that double count them.
Updated · FloorFigures
Short answer
Yes. Under ANSI Z765-2021, finished stairs and landings count on the floor they descend from, up to the area of the floor opening, and the floor beneath them counts on the lower level whatever the headroom. In practice: measure each level around its outside walls, don't subtract the stairwell, and don't add the stairs again. A 3.5 × 12 ft stairwell is 42 sq ft of the upper floor.
Stairs connect two levels, so it's natural to wonder which one they belong to, and both answers people guess at ("neither" and "both, plus the stairs") are wrong. The rule in ANSI Z765-2021 is short. Here it is, the cases it covers, and the mistakes that move a total up or down.
Which floor do stairs count on?
The stair treads and landings count as finished area on the floor from which the stairs descend, but never more than the area of the opening in that floor. Most stairs fill their opening exactly, so the upper floor simply keeps the area of the stairwell. Downstairs, the floor beneath the stairs is part of the lower level's area, whatever the headroom or finish; there is no ceiling-height requirement under stairs.
Put together: in a two-story house where everything is finished and the opening is no bigger than the stairs, each floor's area is just its outline. Nothing is added or subtracted for the stairs.
| Stair | Upper floor (descends from) | Lower level |
|---|---|---|
| Between two finished floors | Treads and landings, up to the opening | Floor beneath the stairs, any headroom |
| Opening larger than the stairs (open well) | Treads and landings only; the rest of the opening is excluded | Floor beneath the stairs |
| Down to an unfinished basement | Main-level finished area, whatever the stairs' finish | Part of the basement level (below grade) |
| Down to a finished basement | Main-level finished area | Part of the basement level (below grade) |
| Up to an unfinished attic | See the 2021 clarification below | Main floor beneath, as usual |
| Up to a room over the garage | Counted as usual, and gives the room the finished connection it needs | Floor beneath, as usual |
| Outside stairs to a deck or porch | Not finished area | Not finished area |
A finished stair also does a second job: it's the continuous finished connection that lets a room over the garage count as finished area. Reach that room only through the garage and it can't be counted on any level. See does a garage count.
Worked example: a two-story house with a basement
A 30 × 40 ft house with two finished floors over an unfinished basement. The main stair fills a 3.5 × 12 ft opening in the second floor. The basement stair sits in a 3.5 × 10 ft opening in the first floor. About 3.5 × 6 ft of the floor under the main stair has less than 7 ft of headroom.
| Level | Calculation | Area |
|---|---|---|
| First floor | 30 × 40, basement stair opening and the floor under the main stair included | 1,200 sq ft |
| Second floor | 30 × 40, main stairwell included | 1,200 sq ft |
| Above-grade finished | 2,400 sq ft | |
| Basement | 30 × 40, unfinished | 1,200 sq ft unfinished below grade |
The mistakes that double count (or undercount) stairs
| Mistake | Above-grade total | Error |
|---|---|---|
| Adding the stairs as a separate area on top of each floor's outline | 2,442 sq ft | +42 sq ft |
| Subtracting the main stairwell from the second floor as open to below | 2,358 sq ft | −42 sq ft |
| Subtracting the basement stair opening because the basement is unfinished | 2,365 sq ft | −35 sq ft |
| Excluding the low floor under the main stair for the 7 ft rule | 2,379 sq ft | −21 sq ft |
The first is the true double count: the stairs are already inside both outlines, so adding them again counts the stairwell twice on one floor. It can happen when someone adds up room sizes from a plan and then adds a "stairs" line to cover the hall. The other three come from applying a good rule to the wrong space: openings are excluded, but a stairwell isn't an empty opening; unfinished basements are separate, but their stairs aren't; and the 7 ft rule doesn't apply under stairs.
When the opening is bigger than the stairs
The "up to the area of the opening" limit matters when a stair wraps around an open well or runs alongside a wider opening. Say the opening upstairs is 10 × 12 ft (120 sq ft), and the treads and landings cover 70 sq ft of it. The upper floor counts the 70 sq ft of stairs and excludes the other 50 sq ft as open to below. Measure the upper floor's outline, subtract the whole opening, then add back the treads and landings. More on openings in open-to-below areas.
Stairs to the basement
Stairs that go down to an unfinished basement count as finished area on the first level, whatever their finish. Stairs to a finished basement count on the first level too, since that's the floor they descend from. Either way, the basement itself is a below-grade level, reported separately from above-grade area. See does a finished basement count.
Stairs to an unfinished attic: the 2021 clarification
The publisher, Home Innovation Research Labs, says the 2021 edition didn't fundamentally change the standard; its main clarifications concern stairs leading to unfinished areas and the sloped-ceiling figure. The publisher's 2020 public draft says finished stairs suitable for year-round use that go up to an unfinished upper area are included in the square footage calculation.
Counting stairs in FloorFigures
FloorFigures is free and runs in your browser. Draw the stairwell on the upper level as a Stairs area and it counts as finished area on the floor it descends from. Overlapping areas are merged, so a stair drawn inside the floor outline is never counted twice. Mark a true opening as Open to below and it's subtracted. With your own areas, the ANSI Z765 GLA calculator keeps above-grade, below-grade and openings separate. For the rest of the rules, see the ANSI Z765 guide, ceiling height rules and how to measure a house.
Questions
- Do stairs count in square footage?
- Yes. Under ANSI Z765-2021, finished stair treads and landings count as finished area on the floor the stairs descend from, up to the area of the opening in that floor. The floor beneath the stairs counts on the lower level. If you measure each level around its outside walls and don't subtract the stairwell, the stairs are already counted correctly.
- Which floor do stairs count on?
- The floor they descend from, which is the upper one. In a two-story house, the main stair counts on the second floor, filling the stair opening. Basement stairs count on the main floor. The floor area underneath any stair belongs to the level below.
- Do basement stairs count as square footage?
- Yes, on the main level. ANSI Z765-2021 counts stairs that descend to an unfinished basement as finished area of the first level, whatever the finish of the stairs. Don't subtract the basement stair opening from the main floor, and don't count the stairs as basement area.
- Does the space under the stairs count?
- Yes, on the lower level. The area beneath stairs counts in that level's finished area regardless of headroom or finish, and ANSI Z765 has no ceiling-height requirement under stairs. Don't exclude the low part under a staircase because of the 7 ft rule.
- Should a stairwell be subtracted as open to below?
- No. Openings to the floor below are excluded, but a stairwell is filled by the stairs, which count on the upper floor up to the area of the opening. Only if the opening is larger than the treads and landings, as with an open well, is the leftover part of the opening excluded.
- Are stairs counted twice in a two-story house?
- Not in the sense of an error. The stairwell counts on the upper floor and the floor beneath the stairs counts on the lower floor. Those are two different floor areas, one per level, which is why a two-story house with a 30 × 40 ft footprint has 1,200 sq ft on each floor. Adding the stairs again as a separate area on top of that would be double counting.