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Approved Document K limits for private stairs in dwellings: maximum rise 220mm, minimum going 220mm, pitch no steeper than 42 degrees, and twice the rise plus the going between 550mm and 700mm. Add 2m headroom throughout. A design starting point; regulations and structural detail belong with a builder or building control.
What The Stair Calculator Works Out
- Number of risers: the floor-to-floor height divided by the maximum permitted rise of 220mm, rounded up, then the height divided by that count for the actual rise per step. Every rise must be identical; varying rises are the classic trip hazard and an automatic fail. A 2,600mm floor height needs 12 risers of about 217mm.
- Total run: one fewer going than risers, times the going. With a 250mm going the example staircase runs 2,750mm horizontally. A generous 280mm going feels better underfoot but eats another 300mm-plus of floor plan.
- Building regs check: Approved Document K for private stairs in dwellings: rise no more than 220mm, going at least 220mm, pitch no steeper than 42 degrees, and twice the rise plus the going between 550mm and 700mm, the formula that makes stairs feel natural to climb.
- Other rules: 2m headroom over the pitch line, handrails at 900 to 1,000mm on at least one side, no gaps a 100mm sphere could pass through, and new or altered staircases are notifiable building work, so run the design past building control or a stair manufacturer before anything is cut.
- Maximum rise you will accept (advanced): lower the 220mm legal maximum to, say, 190mm for a gentler stair; the calculator adds risers accordingly. Many designers aim for 180 to 200mm in a family house.
- Stair width and tread nosing (advanced): sets the cutting size for treads (going plus the nosing overhang, typically 16 to 25mm) and risers. There is no minimum width for a private stair in England, but 800 to 900mm is usual and loft stairs are often narrower.
- String and handrail length (advanced): the string is the diagonal from the bottom of the first riser to the top of the flight, so buy timber at least this long per side. The handrail runs the length of the pitch line plus about 300mm for the end returns.
- Headroom at the trimmer (advanced): enter the stairwell opening length and the floor thickness and the calculator works out the vertical clearance where the pitch line passes under the edge of the floor above. It must be at least 2,000mm; if it falls short, the sub-line gives the opening length that would fix it.
- Floor space taken (advanced): the plan footprint of the flight itself. Allow a further 400mm of clear landing at top and bottom, and remember a door must not open across the bottom step.
- Step table (advanced): cumulative rise and going for each step, ready for marking out the strings or checking a manufacturer's drawing.
Source and further reading: GOV.UK: Approved Document K.