- Appendix AQ relaxes standard stair requirements specifically for tiny homes under 400 sq ft — regular code would eat up too much floor space
- Riser height must fall between 7" and 12" — steeper than a standard home but safer than a ladder
- Two formulas link riser height and tread depth — which one you use depends on whether your height or your floor space is the fixed constraint
- Ceiling under 6’2" at the loft entry? Code requires a landing platform — in a tiny home this almost always applies
- The calculator below handles all the math including subfloor adjustments and landing platform riser, to 1/32" precision
If you’re building a tiny home with a loft and want to pull a permit, your stairs need to meet Appendix AQ of the International Residential Code. This is the section added specifically for tiny homes under 400 square feet. It relaxes the standard stair requirements — regular code would eat up too much floor space in a small build — but you still have to follow specific formulas for rise and run.
This post walks through how the math works and includes a calculator that handles all of it for you.
Appendix AQ Stair Requirements At-A-Glance
| Requirement | Appendix AQ Specification |
|---|---|
| Riser Height | 7" Minimum to 12" Maximum |
| Tread Depth | 7" Minimum (calculated by formula) |
| Stair Width | 20" min (below rail) / 17" min (at rail) |
| Headroom | 6’2" minimum (otherwise requires landing platform) |
The Two AQ Stair Formulas
The code gives you two formulas that link riser height and tread depth. You use one or the other depending on which dimension is fixed in your design.
Riser height has to land between 7" and 12". Below 7" and you end up with too many steps; above 12" starts to feel like a ladder.
Figure Out What’s Fixed First
Before you calculate anything, decide which dimension you’re working from. In most tiny home builds the height is the real constraint since the loft is already framed. But if you’re tight on floor space horizontally, start with run instead.
Fixed height: You know the floor-to-loft distance. Pick a step count, divide to get your riser, then use the tread depth formula to find how much floor space the stairs use.
Fixed run: You have a set amount of horizontal floor before you hit a wall or another room. Divide that by your step count to get tread depth, then use the riser height formula.
Save a Tread: Let the Loft Floor Be the Last Step
Here’s a trick that saves one tread of horizontal run. Instead of building your stairs all the way up to loft floor level, stop one riser short. The act of stepping from the top tread up onto the loft floor counts as the final riser — you don’t need to build a tread for it because the loft floor itself is that tread.
The result: same total height covered, but one fewer tread of floor space used. With 10 steps you get 11 risers of height. The calculator handles both options.
Landing Platform Requirement
Per AQ §AQ104.2.1.4, if the ceiling height at the loft where your stairs connect is less than 6’2", you’re required to build a landing platform at the top of the staircase. In a tiny home this almost always applies. The landing platform is a wider step that you stand on before stepping up onto the loft floor — it gives you room to orient yourself under the low ceiling.
| Landing Platform Spec | Requirement |
|---|---|
| Depth (nosing to loft edge) | 18" to 22" |
| Width | At least 20" |
| Platform riser to loft floor | 16" to 18" |
Getting the platform riser to land in the 16"–18" range is the tricky part — it depends on step count and total height. The calculator flags it and lets you adjust step count until it’s in range.
The Calculator
Walk through the steps below. Inputs are in 1/16" increments; results are given to 1/32" so rounding error across the full flight stays well within the 3/8" code tolerance for riser consistency.
No: The top tread of your stairs is level with the loft floor. N steps = N risers.
Notes on the Results
If you started on subfloor, the calculator subtracts your finish floor thickness from the total rise before dividing. That gives you the riser height the stairs actually need to cover once the floor is down. The bottom riser is then built taller by that same amount — cut it, put the floor down, and every riser in the flight reads the same height.
If the step count is too high or too low for your height, the calculator will tell you which direction to go and block the adjuster button that would make things worse. Keep pressing the other button until the riser height falls in the 7"–12" range.
If you have a landing platform, watch the platform riser badge. Use +/− to adjust step count until it turns green. Usually one or two steps in either direction does it.
Results are shown to 1/32". The rounding note on the results screen shows exactly how much the bottom riser has to absorb and confirms it’s within the 3/8" code tolerance.
Height mode gives you the most complete results. Run mode with a landing required can’t compute the exact platform riser without knowing total height — the calculator will flag this in the notes.
If you have any questions leave a comment below.
Appendix AQ Stair FAQ
What is Appendix AQ?
Appendix AQ is a section of the International Residential Code (IRC) specifically written for tiny houses under 400 square feet. It allows for steeper stairs and ladders that would not be legal in a standard full-sized home, while still providing a framework for safe construction that satisfies permit requirements.
What is "Rise" and "Run"?
Rise is the total vertical distance from the floor to the loft surface. Run is the total horizontal distance the staircase covers on your floor. In most tiny home builds the rise is the fixed constraint since the loft is already framed — but if floor space is tight, you may need to start from run instead.
Do I need a landing platform?
If your ceiling height at the loft entry is less than 6’2", code requires a landing platform. This platform acts as an intermediate step that gives you safe headroom before you fully transition onto the loft floor. In a tiny home this almost always applies — measure carefully from the loft finished floor surface to the ceiling directly above the stair connection point.
What is the minimum stair width?
Under Appendix AQ, the stair must be at least 20" wide below the handrail. At or above handrail height the minimum drops to 17". The handrail itself can project into this space up to 4-1/2" on each side.
What is the maximum riser height?
The maximum riser height allowed under Appendix AQ is 12". For comparison, standard home stairs are capped at 7-3/4". So AQ stairs are significantly steeper — that’s by design, since a tiny home loft doesn’t have the floor space to run a gentler flight. The minimum is 7"; below that you end up with too many steps for the available run.
What is the landing platform riser range and why does it matter?
The riser from the landing platform up to the loft floor must be between 16" and 18". This is the one non-negotiable dimension that can require you to iterate on your step count — because it depends on the relationship between your total rise and the number of regular risers below the platform. The calculator’s +/− adjuster makes it straightforward: just watch the badge and stop when it turns green.
