Loft Conversion Stairs: Where They Go, Part K Rules, and the Room You Might Lose

Every loft conversion takes something from the floor below. The stair decides what.

It sets where you arrive in the loft, and therefore where the dormer has to give you height. It sets where the trimmers and floor steels go around the opening. And it decides whether you keep the box room. Move it after signing and the steels and dormer get redesigned around the new position; one contractor's own guide warns that a late stair design can force a redraw of the whole scheme.

The loft company draws the stair. You live with what it takes out of your house. Make the position your decision before you sign, while changing it costs a drawing revision and nothing else.

These rules are for England. Wales publishes its own Approved Document K with different paragraph numbers, and Scotland's Building Standards Technical Handbooks treat stairs differently, including a width rule England doesn't have.

Three places a loft stair can go

Over the existing stair

The efficient option, and the one designers reach for first. The new flight rises above the old one, so the first floor loses little: the stairwell is already circulation space. It also keeps the new stair inside the existing stair enclosure. Hampshire's Building Control Partnership tells homeowners the new stair should continue from the existing stairwell where possible.

The catch is height. The roof slope, a purlin or the build-up of the new floor can take headroom away near the top of the flight, which is why the stair and the dormer get designed together.

Off the landing

Where the old stairwell has no usable height above it, the new stair often starts from the first-floor landing and turns up into the loft. Landings are rarely generous, so this usually takes a slice of whatever sits beside it: a cupboard, or part of the box room.

Watch the doors. Part K wants a landing at the top and bottom of every flight, at least as wide and as long as the narrowest part of the flight (paragraph 1.20), kept clear of permanent obstructions. A door may swing across the landing at the foot of a flight only as Diagram 1.8 shows, leaving at least 400mm of landing beside the swing (paragraph 1.24). A loft stair that arrives hard against a bedroom door is where that bites.

Through a bedroom or the box room

When neither works, the stair comes out of a room. In a three-bedroom house that's usually the box room, sometimes a strip of a larger bedroom.

It isn't banned. But the loft stair is part of the fire escape route, so a stair through a bedroom becomes a stair inside its own fire resisting enclosure: partitions of at least REI 30 and a fire door of at least E 20 (Approved Document B Volume 1, paragraph 2.21). Some councils' local guidance goes further and expects the new stair to rise from the existing landing, not a room. Where the flight passes over an existing room, building control guidance asks for its underside to be clad for fire resistance too.

Then look hard at what's left. If the stair lands in the middle of the back bedroom, it may stop being a double. If it eats most of the box room, check a single bed still fits, because a buyer may not count a room that won't take one. Homeowners on property forums worry about exactly that, and they're right to.

No reliable published figure exists for how much floor a loft stair takes; it depends on the flight, any turn and the landings. So don't accept a verbal estimate. Ask for a to-scale plan of the floor below with the stair drawn in, then draw your own bed and wardrobe on it.

The Part K numbers, in plain English

Approved Document K, paragraph 1.3 and Table 1.1, sets the geometry of a private stair, meaning one inside a single house. Four numbers do most of the work.

• Rise, the height of each step: between 150mm and 220mm.
• Going, the tread depth measured nosing to nosing: between 220mm and 300mm.
• Pitch: no steeper than 42°.
• Twice the rise plus the going: 550-700mm, the relationship that keeps a step comfortable to climb.

Any rise in the range can pair with any going in the range, provided the pitch and 2R + G still work. What you can't do is take the tallest rise with the shallowest going. That combination is steeper than the pitch limit, and it's exactly where a tight loft tempts a designer.

42°

The steepest a private stair may be under Approved Document K, Table 1.1 note 1. The loft-specific concession in Part K covers headroom, not pitch.

Every step in a flight has to match, with level treads and the same rise and going throughout (paragraph 1.5). That rules out the familiar fudge of trimming the bottom step when the finished floor levels turn out different from the drawing. Winders are allowed if consecutive tapered treads share the same going, with, on a stair under 1m wide, at least 50mm of tread at the narrow end (paragraphs 1.25 to 1.27 and Diagram 1.9). Open risers need treads that overlap by at least 16mm, and gaps a 100mm sphere can't pass through (paragraph 1.9).

The note to Table 1.1 gives an existing house a little room: where dimensional constraints make the table impossible, an alternative can be agreed with the building control body. Have that conversation before the drawings are final. It isn't a get-out to discover on site.

The Building Safety Regulator opened a call for evidence on Part K in October 2026. It proposes no policy changes, and the current guidance stays in force meanwhile.