Summary
- Part K sets the geometry of safety: private stairs at maximum 42° pitch (rise ≤220 mm, going ≥220 mm), 2 m headroom (relaxed to 1.9/1.8 m for loft conversions), handrails at 900 mm, and guarding at 900 mm internally / 1,100 mm externally resisting 0.36 kN/m.
- Loft conversion stairs are the classic Part K battleground — space-saver (alternating tread) stairs are only acceptable for a single habitable room where a conventional stair genuinely cannot fit.
- Glazing in critical locations (below 800 mm generally, below 1,500 mm in and beside doors) must be safety glass to BS EN 12600 or protected — record it on the window/door schedule.
What the Building Safety Act requires for Part K on non-HRB domestic jobs
Requirements K1–K6 cover stairs, ladders and ramps; falling protection; vehicle barriers; and glazing impact safety. On domestic work the stair is where design ambition meets regulation: every loft conversion lives or dies on whether a compliant stair can be threaded into the plan with 2 m headroom (1.9 m at centre / 1.8 m at the low side is accepted for loft conversions where the stair rises into the roof).
As Principal Designer (BSA), your evidence is dimensional: a stair section proving pitch, rise/going and headroom at Stage 3, guarding heights and baluster spacing (no 100 mm sphere passing through) on the Stage 4 details, and safety glazing marked on the fenestration schedule. These are exactly the items Building Control measures on site, so the drawing record protects you when the joiner improvises.
Note the boundary with Part M: Part K governs the safety geometry of private stairs, while Part M layers access requirements on top. Where both apply, the more onerous provision wins — design to the combined envelope, not to one document.
Key design risks and evidence needed (RIBA Stages 2–4)
- Stage 2: prove the loft stair position with a quick section before promising the client a third bedroom; flag any level changes needing guarding.
- Stage 3: full stair section — pitch ≤42°, 2 m/1.9 m/1.8 m headroom as applicable, landings clear of door swings; guarding to all drops over 600 mm.
- Stage 4: baluster spacing (100 mm sphere rule), handrail height 900 mm (both sides where the stair is ≥1 m wide), guarding load case 0.36 kN/m for domestic, safety glazing locations marked to BS EN 12600 class, and manifestation for any large glazed screens.
Standards and references you should be citing
- BS 5395-1 — stairs: code of practice for straight and helical flights (design detail beyond AD K).
- BS EN 12600 — pendulum impact classification for safety glass (specify class 1(C)1, 2(B)2 or 3(B)3 as appropriate).
- BS 6180 — barriers in and about buildings (guarding design loads and heights).
- BS 6262-4 — glazing in buildings: safety related to human impact (critical location rules).
Domestic stair and guarding dimensions at a glance
| Element | Requirement (private dwelling) | Where it bites |
|---|---|---|
| Stair pitch | Maximum 42°; rise ≤220 mm, going ≥220 mm | Loft conversions, tight plans |
| Headroom | 2.0 m; loft conversions 1.9 m centre / 1.8 m edge | Stairs rising into roof slopes |
| Handrail | 900 mm above pitch line; both sides if width ≥1 m | All stairs with a drop |
| Guarding (internal) | 900 mm, no 100 mm sphere gap, 0.36 kN/m | Landings, galleries, open wells |
| Guarding (external/Juliet) | 1,100 mm | Balconies, terraces, Juliet balconies |
| Safety glazing | Below 800 mm; below 1,500 mm at doors and side panels | Glazed doors, low-level glazing, wet rooms |
Common pitfalls that catch out sole practitioners
- Space-saver stairs proposed for a two-room loft conversion — they are only acceptable for a single habitable room and Building Control will reject the layout late.
- Winders at the foot of a stair with the 50 mm minimum going at the newel ignored, failing on site measurement.
- Juliet balcony guarding at 900 mm instead of 1,100 mm because the internal figure was reused.
- Toughened glass assumed everywhere — laminated may be needed where containment matters (barriers, overhead), and the BS EN 12600 class should be stated, not implied.
Frequently asked questions
What headroom is acceptable on a loft conversion stair?
The general requirement is 2 m measured vertically from the pitch line. For loft conversions, Approved Document K accepts 1.9 m at the centre of the stair width reducing to 1.8 m at the side where the stair passes under the roof slope. Prove it with a drawn section — it is the single most common loft feasibility killer.
When are alternating tread (space-saver) stairs allowed?
Only for access to a single habitable room (plus, at most, an associated bathroom/WC that is not the only one in the dwelling), and only where there is genuinely not enough space for a conventional stair. They need handrails both sides and slip-resistant treads.
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