Loft Conversion Stairs: Part K Building Regulations

Q: Can a loft stair really go over the existing stair?

Usually yes, and it is the most space-efficient layout — stacking the new flight directly above the existing one keeps the footprint small. The constraint is headroom at the top of the existing stair and at the head of the new run where it meets the roof. This is exactly where the loft concession (1.9 m centre / 1.8 m edge) is used. The position of the new run is set so it reaches the loft floor before the sloping ceiling drops below that line.

Q: Is a space-saver stair allowed in my loft?

Only as a last resort. An alternating-tread stair is permitted where a conventional stair genuinely will not fit *and* the loft serves a single habitable room (plus optionally a bathroom/WC that is not the dwelling's only WC). Building control will expect evidence that a normal or winder stair was not possible. If the loft has two bedrooms, a space-saver stair is not acceptable.

Q: Do I need fire doors throughout the house?

You normally need fire doors onto the protected stairway from habitable rooms, because the loft conversion turns the stair into the protected escape route for a three-storey house. That commonly means new FD30 doors to rooms opening onto the stair on the lower floors as well, not just the loft. The exact extent depends on the layout and the fire strategy agreed with building control.

Q: What is the maximum pitch for a loft stair?

42°, the same as any private domestic stair. There is no steeper allowance for loft conversions on pitch — the concession loft conversions get is on headroom, not pitch. If the run is too steep at 42°, you must lengthen it, add a turn, or (conditions permitting) use an alternating-tread stair.

Q: Why does adding a loft room trigger fire upgrades downstairs?

Because escape distance and storey count change. A two-storey house lets occupants escape quickly; a third storey is further from a final exit and needs a protected route to give people time to get out through smoke. Part B therefore requires the stair to be enclosed and fire-resisting, with fire doors and often escape windows — protections the original two-storey house did not need.

Summary

The staircase is usually the hardest part of a loft conversion to get right, because it has to satisfy two different sets of rules at once and there is rarely enough space for either. Part K governs the geometry — how steep the stair can be, how deep and tall each step is, and how much headroom you have where the new stair rises into the existing roof. Part B governs fire safety — because adding a habitable loft normally creates a third storey, the staircase becomes a protected escape route that must hold back fire and smoke long enough for people to get out.

The two pinch points are headroom and footprint. A standard stair wants 2 m of clear headroom all the way up, but a loft stair rises into a sloping roof where the head of the run runs out of height. Part K allows a specific concession for loft conversions — 1.9 m at the centre of the stair reducing to 1.8 m at the side — that no other staircase gets, precisely because of this. The footprint problem is that a full-going stair eats floor space on the landing below; where there genuinely is not room, space-saver (alternating-tread) stairs are permitted, but only under strict conditions and only where the loft serves a single habitable room.

This article covers the Part K geometry (pitch, rise, going, the 2R+G rule, headroom and the loft concession), when alternating-tread stairs are allowed, handrail requirements, and the Part B fire-escape rules that turn the stair into a protected route — fire doors, the 30-minute enclosure, and escape windows. It is written for UK builders and designers working to the current Approved Documents.

Key Facts

Building Regulations Part K (Protection from falling, collision and impact) — governs stair geometry: pitch, rise, going, headroom and guarding.
Maximum pitch — 42° for private (domestic) stairs. A loft stair must not exceed this even when space is tight.
Maximum rise — 220 mm per step for a private stair.
Minimum going — 220 mm per step (the horizontal tread depth) for a private stair.
2R + G rule — twice the rise plus the going should fall between 550 mm and 700 mm for a comfortable, regular stair.
Standard headroom — 2 m of clear headroom measured vertically above the pitch line of the stair.
Loft conversion headroom concession — where 2 m is not achievable, a reduced headroom of 1.9 m at the centre of the stair width, reducing to 1.8 m at the edge, is acceptable for loft conversions specifically.
Minimum stair width — no statutory minimum width for a private stair under Part K, but ~600 mm minimum is practical and usable.
Consistent steps — all rises in a flight must be equal, and all goings equal; no single odd step.
Handrail — required on at least one side if the stair is less than 1 m wide, on both sides if 1 m or wider; handrail height 900 mm to 1000 mm above the pitch line.
Guarding — required to landings and the open sides of flights; a 100 mm sphere must not pass through any gap (no-foothold guarding for children).
Alternating-tread (space-saver) stairs — permitted only where there is insufficient space for a conventional stair AND the stair serves a loft containing a single habitable room (plus optionally a bathroom/WC).
Building Regulations Part B (Fire safety) — a loft conversion creating a third storey requires a protected stair enclosure forming the escape route.
Protected stairway — the stair enclosure must provide 30 minutes' fire resistance, with fire doors (FD20/FD30) to habitable rooms opening onto it.
Fire doors — habitable rooms opening onto the protected stair typically need 30-minute fire doors (FD30) with self-closers.
Escape window — habitable rooms in the loft (and sometimes existing floors) require an openable escape window meeting Part B size and sill-height rules where the protected-route strategy requires it.
Sprinklers / alternative strategy — where a protected stair cannot be achieved, an alternative approach (e.g. sprinkler system, open-plan concession) may be agreed with building control.

Quick Reference Table

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Parameter	Requirement	Reference
Maximum pitch	42°	Part K
Maximum rise	220 mm	Part K
Minimum going	220 mm	Part K
2R + G	550–700 mm	Part K
Standard headroom	2.0 m	Part K
Loft headroom (concession)	1.9 m centre / 1.8 m edge	Part K
Handrail height	900–1000 mm	Part K
Guarding gap	< 100 mm sphere	Part K
Alternating-tread	Single habitable room only	Part K
Stair enclosure FR	30 minutes	Part B
Fire doors to rooms	FD30 (self-closing)	Part B
Escape window	Per Part B size/sill	Part B

Detailed Guidance

Part K geometry: pitch, rise and going

Every step in a domestic stair is defined by its rise (height) and going (tread depth). Part K caps the rise at 220 mm and sets the going at no less than 220 mm, with the whole flight kept to a maximum pitch of 42°. Within those limits the comfort check is the 2R + G rule: twice the rise plus the going should land between 550 mm and 700 mm. A stair with 200 mm rise and 230 mm going gives 2×200 + 230 = 630 mm — comfortable and compliant.

STEP GEOMETRY CHECK
Rise <= 220mm?           ----> if no, redesign
Going >= 220mm?          ----> if no, redesign
2R + G between 550-700?  ----> if no, adjust rise/going
Overall pitch <= 42?     ----> if no, lengthen the run
All rises equal & all goings equal? ----> must be yes

The trap on a loft job is that a steep roof and a small landing tempt you toward a steeper pitch — but 42° is a hard ceiling for a private stair. If the geometry will not fit, the answer is a longer run, a winder/turn, or (as a last resort and only if the conditions are met) an alternating-tread stair.

Headroom and the loft conversion concession

Standard Part K wants 2 m of clear headroom measured vertically from the pitch line. On a loft conversion the stair climbs into a sloping ceiling, so the top of the run is exactly where headroom runs out. Recognising this, Part K gives loft conversions a unique concession: where the full 2 m cannot be achieved, 1.9 m at the centre of the stair width reducing to 1.8 m at the side is acceptable.

LOFT HEADROOM (cross-section at the tight point)

      sloping ceiling
        \
         \      1.9m at centre
          \   ___________
           \ |           |  1.8m at edge
            \|           |
   ----------+--pitch line-+----------
              stair width

This concession applies only to loft conversions — you cannot use it to justify low headroom on an ordinary new stair. Design the stair position and the trimming of the roof structure so the run reaches the loft floor before the ceiling drops below the concession line.

Alternating-tread (space-saver) stairs

Where a conventional stair genuinely cannot fit, an alternating-tread (paddle) stair is allowed — but the conditions are strict. It is permitted only where there is not enough space for a conventional stair and the stair serves a loft containing a single habitable room (it may also serve a bathroom or WC, provided that is not the only WC in the dwelling). Each tread has half the step cut away, with the cut sides alternating left/right, so the user must lead with the correct foot. Handrails are required on both sides. It is a last resort, not a default — building control will expect to see that a conventional or space-efficient winder stair was genuinely impossible first.

Handrails and guarding

A handrail is required on at least one side where the flight is under 1 m wide, and on both sides where it is 1 m or wider, at a height of 900–1000 mm above the pitch line. Open sides of flights and landings need guarding designed so a 100 mm sphere cannot pass through any gap — this stops a small child slipping through or getting a foothold to climb. The guarding must be robust enough to resist the loadings in Part K.

Part B: the protected escape route

This is the part that surprises homeowners. Converting a loft normally creates a third storey, and Part B then requires the staircase to be a protected escape route: an enclosure giving at least 30 minutes' fire resistance that leads from the new floor down to a final exit. In practice this means:

The stair enclosure (walls and ceiling) must achieve 30 minutes' fire resistance.
Habitable rooms opening onto the protected stair need fire doors (typically FD30) to hold back fire and smoke; many require self-closers.
The route must lead to a final exit without passing through a risk room such as an open-plan kitchen, unless an alternative strategy is agreed.
Where a fully protected stair cannot be achieved (for example with open-plan ground floors), an alternative approach — often a domestic sprinkler system, or additional escape provision — is agreed with building control.
Escape windows are required to habitable rooms where the strategy depends on them, sized and positioned to Part B (an unobstructed openable area and a sill within the permitted height range).

ESCAPE ROUTE (3-storey loft conversion)

  Loft room --[FD30]--+
                      |
  Protected stair (30-min enclosure)
                      |
  First floor room --[FD30]--+
                      |
  Hall --[FD30 to risk rooms]--> FINAL EXIT (front door)

Get the fire strategy agreed with building control early — it dictates door specifications, the stair position, and whether escape windows or sprinklers are needed, all of which affect the layout and cost before a single tread is cut.

Frequently Asked Questions

Can a loft stair really go over the existing stair?