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 at the bottom 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 (under rail) / 17" Min (at rail) |
| Headroom | 6' 2" Minimum (otherwise requires landing) |
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 you need to decide which dimension you're working from:
Fixed height
You know the floor-to-loft distance. Pick a step count, divide to get your riser, then use Formula 1 to find tread depth. Tread depth determines 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 Formula 2 for riser height.
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.
Save a Tread: Let the Loft Floor Be the Last Step
Here's a trick that saves you one tread of horizontal run. Instead of building your stairs all the way up to loft floor level, you stop one riser short. The act of stepping from your top tread up onto the loft floor counts as the final riser — you just 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 essentially 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.
The landing platform specs:
- Depth from nosing to loft edge: 18" to 22"
- Width: at least 20"
- Riser from landing platform up to loft floor: 16" to 18"
Getting that platform riser to land in the 16"–18" range is the tricky part. It depends on the number of steps and your total height. The calculator below figures this out and flags it if you're out of 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 under 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 Tiny House Stair FAQ & Glossary
What is Appendix AQ?
Appendix AQ is a section of the International Residential Code (IRC) specifically written to provide safe but flexible building standards 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.
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.
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.
What is the minimum stair width?
Under Appendix AQ, the stair width must be at least 20 inches wide below the handrail. At or above the handrail height, the width can be reduced to a minimum of 17 inches.
What is the maximum riser height?
The maximum riser height allowed is 12 inches. Standard home stairs are usually capped at 7.75 inches, so Appendix AQ stairs are significantly steeper.
