Where the Loft Staircase Goes, and What It Costs You Downstairs

Almost every loft conversion that gets abandoned is abandoned over the stairs. Not the roof, not the money, not the planning position. Somewhere in the design the flight turns out to need about three metres of the floor below, and the only place with three metres to spare is a bedroom. This guide sets out the geometry that produces that number, the headroom rule everybody quotes and half the internet quotes backwards, and what the flight takes from the floor it starts on.

The geometry, and the length it puts on your plan

Four numbers from Approved Document K, and between them they decide how much floor the stairs eat.

A stair inside a house is what the regulations call a private stair, and it has its own set of limits. Each step may rise between 150mm and 220mm. Each going, the horizontal depth of the tread from nosing to nosing, must be between 220mm and 300mm. Twice the rise plus the going should come to between 550mm and 700mm, which is a comfort rule rather than an arbitrary one. And the pitch of the whole flight may not exceed 42 degrees.

Now run a real house through it. Take a floor to floor height of 2.7 metres, first floor finished level up to loft finished level, which is ordinary once the new floor structure is in. Divide by the 220mm maximum rise and you need thirteen risers, so each one is about 208mm. Twelve goings follow.

The tempting move is to use the 220mm minimum going and keep the flight short. It does not work, and the reason is the one people miss: at a 208mm rise, a 220mm going puts the pitch at over 43 degrees. The 42 degree cap bites before the going minimum does, and it forces the going out to about 231mm. Twelve of those is 2.77 metres of straight run, before you have added a landing at either end.

That is the whole problem in one figure. Nearly two and a half metres of usable roof space costs you nearly three metres of the floor underneath, and on a semi the first floor landing is rarely more than two.

The headroom rule, and the relaxation quoted backwards

Two metres over the flight, with one specific loft relaxation, and the two figures in it are frequently swapped.

The standard requirement is 2 metres of clear headroom measured vertically above the pitch line of the stairs, and the same at the landing where you arrive. Because a new flight rises through the existing roof structure, cut or trussed, this is what usually decides where the stairs can go rather than what they look like.

There is a relaxation written specifically for loft conversions, and it is worth getting right. Where there is not enough space to achieve 2 metres, Approved Document K accepts reduced headroom of 1.9 metres measured at the centre of the stair width, falling to 1.8 metres at the side of the stair. That way round. You will find it stated the other way in plenty of places, including in descriptions written by people who should know, and the reversal matters, because 1.8 metres in the middle of a flight is a duck and 1.9 at the edge is nothing you would notice.

Where the ceiling above the flight is a slope rather than a soffit, as it is on a hipped roof until the hip is built up to a gable, the height falls away across the width of the stair, which is exactly the situation the relaxation was written for. Where the shortfall is at the top of the flight rather than over it, a dormer sitting above the stair head is often the fix, which is one reason a rear dormer is sometimes positioned to solve the stair rather than to improve the bedroom.

Width, handrails and guarding

There is no minimum width for a stair inside a house, which surprises almost everybody.

Approved Document K sets no minimum width for a private stair. The only width figure it gives for a dwelling covers a stepped change of level within the entrance storey on a severely sloping plot, which is not this. What sets the width in practice is furniture, the handrail rule, and how the flight feels to somebody carrying a basket of washing.

A handrail goes at 900mm to 1000mm above the pitch line, and where the stairs are 1000mm or wider a handrail is needed on both sides. Guarding to a stair, landing or the edge of a floor in a house is 900mm high. If the house may be used by children under five, guarding must be built so a 100mm sphere cannot pass through it and so it cannot readily be climbed, which rules out the horizontal wire and the wide baluster spacing that look good in photographs.

Open risers are allowed in a house provided the treads overlap by at least 16mm and a 100mm sphere cannot pass through the gap. That combination is what makes an open tread loft stair look heavier in reality than in the visual you were shown.

The landing is what actually costs you the bedroom

Everyone budgets for the flight. The landings at each end are what push it into a room.

At the top and bottom of every flight there has to be a landing at least as wide and as long as the narrowest part of the flight. On a 850mm stair that is 850mm at each end, added to 2.77 metres of run. A landing is allowed to include part of the floor of the building, which is the concession that saves most of these jobs, because the existing first floor landing can serve as the bottom landing rather than a separate area being carved out for it.

Doors are where it goes wrong. A door may swing across a landing at the bottom of a flight only in a defined arrangement that keeps at least 400mm of clear landing beyond the swing. Cupboard and duct doors may open onto a landing at the top of a flight, but only where they are kept shut in normal use, again with 400mm clear. On a 1930s semi the first floor landing usually has four doors and an airing cupboard opening onto it, and putting a new flight through the middle of that is how a landing turns into a corridor where two doors foul each other.

The result is usually one of three things. The landing absorbs the flight and nothing is lost. The airing cupboard and the cylinder in it go along with a slice of the smallest bedroom, and the bedroom survives as a single. Or the smallest bedroom becomes the stairwell, the house gains a loft room and loses a bedroom, and the net bedroom count has not moved. That last outcome is not a failure of the builder; it is a fact about the plan, and it should be established on a drawing before anybody is charged for anything.

Winders, and why they are usually the real answer

Turning part of the flight through ninety degrees is how 2.8 metres of stair fits into a two metre landing.

Tapered treads, which most people call winders, let the flight change direction while still climbing. They have their own rules. The going of a tapered tread is measured at the centre of the tread, and on a stair 1 metre wide or more it is measured 270mm in from each side. Consecutive tapered treads must all have the same going, and where a flight mixes straight and tapered treads the tapered ones must not have a smaller going than the straight ones. The narrow end of a tread must still be at least 50mm wide.

What that means on a plan is that a set of three winders at the bottom turns roughly 700mm of straight run into a quarter turn occupying a square, and a straight flight that would have run out through a bedroom wall now runs up alongside the existing stairs instead. Nearly every conversion over an existing staircase on a terrace or a semi does exactly this. It is also the part most often drawn optimistically, because a winder that satisfies the going at the centre line can be genuinely unpleasant at the newel.

Spiral and alternating tread stairs, and what they are actually for

One of them is allowed more freely than people think, and the other solves a different problem from the one it gets reached for.

Spiral and helical stairs are not restricted to particular rooms. Approved Document K simply says design them to the relevant British Standard, and they are used in houses. What limits them is practical: the going is measured on a walk line rather than at the newel, so a compliant spiral needs a bigger diameter than the ones in showrooms, and getting a mattress, a wardrobe or a person who is unwell up one is a real consideration for a room that is going to be a bedroom.

An alternating tread stair, the sort with paddle-shaped treads, is restricted, and tightly. It may be used only in a loft conversion, only where there is genuinely not enough space for a stair meeting the ordinary rules, and only where it serves one habitable room plus, if wanted, a bathroom or WC that is not the only WC in the house. Note what that excludes: a loft with a bedroom and a study, or a bedroom and a second bedroom, does not qualify.

The detail that catches people out is headroom. The construction guidance for an alternating tread stair asks for a minimum clear headroom of 2 metres. So it is not a fix for a roof that will not give you height above the flight, which is exactly the situation people reach for it in. It saves floor area, not height. A fixed ladder is more restricted still: one habitable room, fixed handrails both sides, and only where a proper stair cannot be fitted without altering the existing space.

Where the flight has to land, which is a fire question

The stair is not just a way up. Once the new floor is high enough, it becomes the escape route for the whole house.

The trigger is a height rather than a count of storeys, and the line is a floor more than 4.5 metres above ground level. Above that, escape through a window is no longer accepted, so the staircase has to be a protected route running from the top floor down to a final exit door, with fire doors onto it and a fire resisting enclosure around it.

That reaches back into the stair design in one specific way: the new flight has to arrive into, and discharge into, that protected enclosure. A beautiful flight landing in an open plan kitchen has not solved the problem, it has moved it, and the answers then are a glazed fire screen, a sprinkler or misting system, or a second escape route. All of that is set out on the fire safety and escape guide, and it is worth reading before the stair position is fixed rather than after.

What the opening does to the room below

A stairwell is a hole in a floor, and holes in floors have consequences at the edges.

Forming the opening means cutting the existing ceiling joists and trimming the hole with doubled timbers, and where the opening runs across the span that trimmer is a structural member sized by the engineer rather than chosen on site. The ceiling below comes down along the line of the opening and is made good, which is redecoration in a room nobody planned to touch.

Then the small things that show up later. A radiator or a light switch usually sits where the new balustrade wants to be. The landing loses whatever borrowed light it had from the bedroom door that has now been moved. And a stairwell in the middle of a plan can be turned from the worst-lit spot in the house into the best by a single rooflight over it, which is the cheapest good decision available on the whole job.

Working out whether yours fits

Two measurements and a broom handle, done in twenty minutes, and you will know before you commission a drawing.

Measure the existing first floor landing, end to end, in the direction a flight would run. Then work out your floor to floor height: existing first floor level up to where the new loft floor will sit, which is roughly the top of the new joists above the existing ceiling joists. Mark a broom handle at 2 metres, take it into the loft, and stand it where the top of the flight would arrive.

If the landing has 2.8 metres in a straight line, you are in the easy case. If it has around two metres, you are in the winder case, which is most houses. If it has neither, and the broom handle is under the mark wherever you stand it, the honest answer is that the stair costs a bedroom, and the question becomes whether the room you gain is worth the room you lose. On some plans it plainly is. On a three bedroom terrace, plainly not. We work that out at the survey and say so, and how the rest of the job follows is on the loft conversions page. The figures sit on the cost page.

The dimensions here are taken from the current Approved Document K and describe how a private stair is assessed in general. How they apply depends on your floor to floor height, your landing and your roof, and Building Control take a view on the specific stair rather than on a description of the rules.

Last reviewed August 2026. Planning and Building Regulations change, and your property may differ.