Space-saver (alternating tread) stairs: what they are and when you can have one
Written by Scott Jones, The Stair Guys, independent staircase measuring and sourcing specialists·Last updated
A space-saver stair, also called an alternating tread stair or a paddle stair, has treads cut away so each one is only wide enough for one foot, left then right, which lets the stair climb far more steeply in far less floor space than a conventional flight. It is a genuinely useful answer to a very tight space. It is also widely misunderstood as an upgrade, when it is really a compromise you use because you have to, not because it is better.
When you are actually allowed one
You cannot simply choose a space-saver because it is cheaper or tidier. Building regulations only permit an alternating tread stair where there is genuinely not enough room for a conventional staircase, and typically only in a loft conversion giving access to a single habitable room (it may also serve a bathroom or WC as long as that is not the only room it reaches). It also has to be built properly for the job: fixed firmly, handrails on both sides, and treads you are not going to slip off. If a conventional stair can be made to fit, that is what the regulations expect you to use.
The trade-offs, honestly
A space-saver is steep, and the alternating treads take a bit of getting used to, especially coming down and especially for anyone less steady on their feet. It is not the stair you want to carry a laundry basket or a piece of furniture up. Because it is so often the budget option for a tight loft, it also tends to attract the cheaper end of materials and finish, so the "what is it actually made of" questions matter here as much as anywhere.
None of this makes it wrong. In a loft that genuinely will not take a normal stair, a well-made alternating tread stair is the right answer. Just go in knowing it is the answer to a space problem, not a better staircase.
Before you settle for one
Because a space-saver is only allowed where a conventional stair will not fit, the first question is always whether a normal stair really cannot be made to work, which comes down to the run, the opening and the headroom. Plenty of lofts that look too tight for a proper stair are not, once the space is measured properly rather than judged off a plan. For that, see our guides on staircase headroom and why a staircase that works on the plan can fail in reality. For the full rules, see the UK staircase building regulations guide.
Frequently asked
What is a space-saver or alternating tread staircase?+
It is a steep staircase with treads cut away so each step takes only one foot, left then right, which lets it climb in much less floor space than a conventional stair. It is also called a paddle stair. It is a solution to a very tight space, not a premium option.
When are you allowed a space-saver staircase under building regulations?+
Only where there is genuinely not enough room for a conventional staircase, and typically only in a loft conversion serving a single habitable room (it may also serve a bathroom or WC if that is not the only room reached). It must have handrails on both sides and be built firmly. If a normal stair can fit, that is what is expected.
Are space-saver stairs safe?+
Used as intended, and built properly with handrails on both sides and slip-resistant treads, they are permitted. But they are steep and the alternating treads take care to use, particularly coming down and for anyone less steady, so they are only allowed where a conventional, gentler stair genuinely will not fit.
Can a space-saver stair be the only staircase to a floor?+
It is limited to serving a single habitable room in a loft conversion (plus possibly a bathroom or WC that is not the only room reached). It is not intended as the main stair of a home, which is one reason a conventional stair is expected wherever it can be made to fit. Check the specifics with building control.
Related guides
- Winder staircases: what they are and the rules that govern themA winder staircase turns a corner on tapered treads instead of a flat landing. Here is how winders work, the two rules every winder has to keep, why four winders is a harder job than three, and when a landing is the better answer.
- Types of staircase: the main shapes, and how to chooseThe main staircase types are straight, quarter-turn, half-turn, winder, spiral, helical and space-saver, with open or closed strings on each. The right one is driven by the space you have, the look you want and the budget.
- Mono-stringer and central-spine staircasesA mono-stringer or central-spine staircase carries the treads on a single central beam instead of two side strings, giving a lighter, floating look. The spine can be steel or a thick timber beam, and the treads cantilever out to each side.
- Cut string vs closed string staircase: the difference, and how to chooseA closed string staircase has a solid raking board on the outer edge that hides the ends of the treads, while a cut or open string has that edge cut to the step profile so the tread ends show. Closed is simpler and cheaper, cut is a feature with more joinery.
- Cantilever (floating) staircasesA cantilever or floating staircase fixes each tread at one end only, usually built into the wall, so the treads appear to float with no support on the open side. The wall and a steel support carry the load, so it is a structural, engineered stair rather than standard joinery.
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