An alternating tread stair looks like a stair but climbs like a ladder that lets you keep both feet flat. The treads alternate from side to side, each one wider on one half than the other, so the stair can be pitched steeper while still giving you a tread to stand on.

Where it can be used

In England, Building Regulations guidance allows an alternating tread stair only inside a loft conversion, and only where a normal stair will not fit in the space available. It has to serve one habitable room. It can also serve a bathroom or WC off that room, as long as that bathroom or WC is not the only one in the house.

The dimensions it has to meet

Where an alternating tread stair is used, the guidance sets out fixed figures.

• Headroom: 2 m, measured over the stair
• Handrails: fitted on both sides of the flight
• Maximum rise: 220 mm per step
• Minimum going: 220 mm per step

These aren't independent choices. A flight built to the maximum rise and minimum going is already at the steep end of what the guidance permits, so there's little room to trade one figure against another if the headroom above is also tight.

Before fitting one, it's worth checking what the regulations allow for loft access in your own nation and against your own floor-to-ceiling height. The Building Regulations guide sets out the full provisions nation by nation, and measuring for a loft ladder covers how to take the headroom and landing measurements an alternating tread stair depends on.

Wales works from the same idea as England, though the wording in its own regulations is older. A fixed ladder with handrails on both sides may serve one habitable room in a loft conversion, but only where a normal stair will not fit. Retractable ladders are not acceptable as a means of escape under this provision, so a fixed alternating tread stair is the alternative where space is tight.

Scotland has no equivalent provision

A full-text search of the domestic Technical Handbook finds no mention of an alternating tread stair anywhere in it. Fixed ladders are mentioned, but only for limited-access areas such as plant rooms.

If you're converting a loft in Scotland and a normal stair won't fit the space, the Handbook simply doesn't set out what should go in its place. Ask your local authority's building standards service before you plan the access, and before you buy anything, since this is a case where the written guidance runs out and a local decision takes over.

The provisions for England and Wales, and what counts as building work in each nation, are set out in the Building Regulations guide.