Part K
Approved Document K: Protection from Falling, Collision and Impact
Protection from falling, collision and impact. In England (since April 2013), Part K also absorbed the glazing safety requirements that previously sat in former English Part N: safety glazing in critical locations, opening/cleaning of windows, and impact protection.
Also known as
Part KApproved Document KAD KProtection from Falling, Collision and Impact
What Part K requires for a home project
The Part K requirements and terms the buildwiz guides rely on, with paragraph references where the document gives one. Check the current edition on gov.uk before you build to it.
Limits and figures
- Safety glazing critical zone above door
- Height above finished floor level up to which any glazing in or beside a door (within 300mm of the door edge) must be toughened or laminated safety glass under Approved Document K.
- Safety glazing critical zone alongside door
- Horizontal distance either side of a door within which adjacent glazing falls into the critical safety-glazing zone under Approved Document K. Panels within this strip up to 1,500mm above floor level must be toughened or laminated.
- Safety glazing critical zone in windows
- Height above finished floor level up to which any window glazing must be toughened or laminated safety glass under Approved Document K. Any pane whose lowest edge sits below 800mm falls into the critical zone.
Terms explained
- 100mm sphere rule
- The test that a 100mm diameter sphere must not pass through any opening in stair guarding or between open risers, so a young child cannot get their head through or be held fast. Approved Document K applies it to open risers (para 1.9b), to guarding in buildings children under five may use (para 1.39a) and in Section 3 (para 3.3a). Horizontal balusters make it easier to climb and are discouraged for the same reason.
- Alternating tread stair
- A steep stair with staggered half-treads, so a person uses each foot on alternate steps, which lets it fit in far less floor area than a conventional stair. Approved Document K (para 1.29) allows one only in a loft conversion, only where there is not enough space for a conventional stair, and only where the stair serves one habitable room plus, if desired, a bathroom or WC that is not the only WC in the dwelling. Para 1.30 and Diagram 1.10 require uniform steps with parallel nosings, slip-resistant treads, handrails on both sides, a minimum going of 220mm, a maximum rise of 220mm, a 100mm sphere that cannot pass through open risers, and a full 2m of headroom.
- Bespoke large-format aluminium rooflight
- Made-to-order large-format fixed aluminium rooflights supplied by specialist glazing companies. Substantially different from standard Velux pivot windows: installed by the specialist supplier rather than the roofer, cost £10,000-15,000 for large units, 8-16 week lead time. May require 30-minute fire resistance glazing if over escape routes. A documented 55m² kitchen extension in Oxfordshire used two large units from a specialist supplier.
- BS EN 12150
- Glass in building - Thermally toughened soda lime silicate safety glass. Part 1 specifies definition and description; Part 2 covers product evaluation. Toughened glass marked to BS EN 12150 is the standard safety glazing for doors, low-level windows, side panels, and bath/shower screens: Approved Document K and BS 6262 reference it.
- Egress window
- Openable window meeting Part B minimum dimensions (450mm clear opening, 0.33m² area, bottom of the opening max 1100mm from floor), needed as a means of fire escape in habitable rooms (not kitchens) that do not open onto a hall leading to a final exit (ground storey) or onto a protected stairway (upper storeys up to 4.5m): Approved Document B Volume 1 paras 2.1, 2.2 and 2.10. Locks, with or without removable keys, are allowed.
- Fixed loft ladder
- A non-retractable ladder with fixed handrails on both sides. Approved Document K para 1.32 allows it only for access in a loft conversion containing one habitable room, and only where there is not enough space without altering the existing space for a stair that meets the dwelling guidance. A loft reached only by a fixed ladder usually will not count as a bedroom when selling.
- Handrail
- The rail gripped when using a stair. For a dwelling, Approved Document K para 1.34 asks for the top of the handrail 900mm to 1000mm above the pitch line or floor, a handrail on both sides if the stair is 1000mm wide or more, and allows the handrail to form the top of the guarding if the heights match. An alternating tread stair needs handrails on both sides whatever its width.
- Head height
- The clear vertical space above a floor, stair or landing. On a stair it is measured above the pitch line, not from any single tread. Approved Document K (para 1.11, Diagram 1.3) asks for at least 2m over the flight and landings, and for a loft conversion para 1.13 and Diagram 1.4 allow a reduced 1.9m at the centre of the stair width falling to 1.8m at the side where there is not enough space for 2m. Head height is also the first thing to check when deciding whether a loft can be converted at all.
- Loft staircase
- A fixed staircase serving a loft conversion, designed to Approved Document K: minimum 2m headroom (reducible to 1.8m at the edge of a stair in a loft), maximum 42 degree pitch, maximum 220mm rise and minimum 220mm going. Its position and run length frequently decide whether a loft conversion is feasible at all.
- Open riser stair
- A stair with no vertical riser boards between the treads. Approved Document K para 1.9 allows open risers in dwellings only if the treads overlap by at least 16mm and a 100mm diameter sphere cannot pass through the gap. Alternating tread stairs must also meet the sphere limit (para 1.30e).
- Pitch line
- The sloping line joining the nosings (front edges) of the treads along a flight of stairs. Approved Document K measures headroom above it and the handrail height from it (Diagram 1.3 and para 1.34a). A purlin, rafter or ceiling that dips below the headroom height measured above this line is the usual reason a loft stair fails the headroom check.
- Retractable loft ladder
- A ladder that folds or slides away into the loft hatch. Approved Document K para 1.31 says not to use retractable ladders as a means of escape, so one cannot be the access to a habitable loft room. Fine for storage-only loft access. The permitted ladder in a loft conversion is a fixed ladder (para 1.32).
- Safety glazing
- Toughened or laminated glass required by Part K in critical locations: wall panels up to 800mm from floor, door glazing up to 1500mm from floor, and door side panels.
- Space-saver staircase
- An alternating-tread staircase with paddle-shaped treads that allows a steeper pitch in a tight footprint. Approved Document K permits it only for access to a single habitable room (optionally plus an en-suite or WC) in a loft conversion, and never as the main stair of a dwelling.
- Space-saver staircase kit
- A packaged alternating-tread stair, typically twelve cut-away treads with handrails, sized to a stated floor-to-floor height range and opening size. Retail examples need an opening of roughly 66cm by 140cm. The product that the alternating tread stair rules of Approved Document K apply to, so it is only suitable where those rules permit it.
- Spiral stair
- A stair that winds round a central column (spiral) or an open well (helical). Approved Document K para 1.28 says to design them in accordance with BS 5395-2. Some cost guides say a spiral is not suitable as the main stair to a habitable loft room, so it is a decision to agree with building control before relying on one.
- Stair going
- The horizontal depth of a step, measured from the front edge (nosing) of one tread to the nosing of the next (Approved Document K Diagram 1.1). Table 1.1 allows 220mm to 300mm for a private stair, and the going must be the same on every step of a flight (para 1.5).
- Stair guarding
- The protective barrier at the open side of a flight or landing, usually called the balustrade or banister. Approved Document K requires guarding where there is a drop of more than 600mm (para 1.41), at 900mm high on stairs and landings in a single family dwelling (Diagram 3.1), with openings that a 100mm sphere cannot pass through and a design children cannot readily climb (paras 1.38 to 1.39). A loft stair's open landing edge counts.
- Stair landing
- The level platform at the top or bottom of a flight. Approved Document K para 1.20 asks for a landing at the top and bottom of every flight at least as wide and as long as the smallest width of the flight, kept clear of permanent obstructions (para 1.21). At the bottom of a flight a door may swing across it only as shown in Diagram 1.8. A loft stair that arrives against a bedroom door or opens onto the roof slope is where landing and headroom problems appear.
- Stair pitch
- The angle of a stair measured from the horizontal along the pitch line. Approved Document K Table 1.1 note 1 gives a maximum of 42 degrees for a private stair. It follows from the rise and going: the largest rise (220mm) with the smallest going (220mm) gives about 45 degrees, which is over the limit, so the extremes cannot be used together. Not the same as roof pitch.
- Stair rise
- The vertical height from the top of one tread to the top of the next. Approved Document K Table 1.1 allows 150mm to 220mm for a private stair, and requires every step in a flight to have the same rise (para 1.5). Trimming the bottom riser to suit a floor finish breaks that uniformity.
- Two rise plus going rule
- The comfort relationship quoted in Approved Document K Table 1.1: twice the rise plus the going should come to between 550mm and 700mm. It is a check on a stair that otherwise meets the minimum and maximum rise and going.
- Winder stair
- A stair that turns a corner using wedge-shaped (tapered) treads instead of a flat landing. Approved Document K paras 1.25 to 1.27 apply the normal rise and going rules, with the going measured at the centre of the tread, consecutive tapered treads sharing the same going, and at least 50mm tread width at the narrow end where the stair is under 1m wide (Diagram 1.9). Used where a loft stair has to turn in a tight space. Some building control teams cap the winders in one quarter turn at three.
Pages where Part K comes up
10 guides reference Part K, grouped by build phase.
Planning
Pre-construction
Specification
Want the full picture?
The buildwiz build guides walk through a UK extension in sequence and link to Part K (and every other Approved Document) at the exact stage it bites. That is usually a more productive way to use this material than reading the document cover to cover.