Stair Building Regs Calculator (UK Part K)

UK stair building regulations (Approved Document K)

Under Approved Document K of the Building Regulations, private staircases in England must meet specific dimensional limits to be safe and compliant: riser height between 150mm and 220mm, going (tread depth) between 220mm and 300mm, and a maximum pitch of 42 degrees. All risers must be equal in height and all goings equal in depth throughout the flight: inconsistent steps are a common cause of falls and building control rejections.

The 2R + G comfort formula

Beyond the hard limits, building control also checks the 2R + G formula (twice the riser height plus the going), which should fall between 550mm and 700mm. This approximates a natural walking stride on a slope: steep stairs need shorter goings, shallow stairs need longer ones. Most UK builders aim for a riser around 175mm and a going of 250-275mm, giving a comfortable pitch near 34 degrees, comfortably inside all Part K limits.

Why this matters for loft conversions and tight spaces

Loft conversions are the most common place UK homeowners hit stair compliance problems: floor-to-floor rise is fixed by the existing house, and available going is often limited by the room layout below. If your calculation shows a fail, the usual fixes are: use a winder or spiral staircase to save space (different regs apply: Part K sets slightly different rules for tapered treads), reduce rise by lowering the loft floor if structurally possible, or extend the going into an adjacent room. Once your stair fits, use our Joist Span Calculator to check the floor structure: see the related tools below to confirm your loft conversion itself does not need full planning permission.

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Headroom, width, and nosing requirements under Part K

Approved Document K sets 2000mm minimum clear headroom measured vertically above the pitch line for all domestic stairs. Loft stairs that pass through a hatch are exempt only if the hatch is directly above the top landing - any offset reinstates the full 2000mm rule. Stair width has no fixed minimum under Part K for private dwellings, but Building Control routinely enforces 800mm clear between handrails as a practical standard drawn from BS 5395-1. Open risers are permitted, but each nosing must overlap the tread below by at least 16mm to prevent a 100mm sphere passing through - the same 100mm sphere test that governs balustrade spindle spacing.

Nosing profile matters beyond the overlap. A maximum 25mm overhang is permitted; beyond that, the leading edge becomes a trip hazard rather than a slip reducer. Nosings must contrast visually with the tread for Part M compliance on any stair serving a habitable space - a 30 LRV (Light Reflectance Value) contrast is the recommended minimum, though Part K itself does not specify LRV numbers directly.

Handrail heights and balustrade loading

Part K requires handrails between 900mm and 1000mm above the pitch line on both sides when the stair is wider than 1000mm. For stairs narrower than 1000mm, one handrail suffices. The handrail must be continuous and graspable - circular sections of 40–45mm diameter or oval profiles within that grip envelope satisfy BS 8300. Balustrades must resist a 0.36 kN/m horizontal load at handrail height for domestic use; commercial stairs step up to 0.74 kN/m under BS EN 1991-1-1. This load figure is not part of the calculator's output - a structural engineer must verify the fixing specification independently.

Glass balustrades in domestic settings need a minimum 10mm toughened or 8.8mm laminated pane to satisfy the Part N glazing requirements that apply wherever glazing sits within 800mm of a floor or landing.

What this calculator does not cover

The tool checks rise, going, and the 2R+G formula only. It does not calculate structural loads, string or trimmer joist sizing - use the joist span calculator for that. It does not account for fire-protected escape stairs under Part B, which impose a maximum 190mm rise and 250mm going regardless of the comfort formula. External stairs are also outside scope: Part K Section 3 applies different pitch limits (maximum 38°) and tread depth rules for external access. Always submit a Building Regulations Full Plans application - not a Building Notice - for any structural stair alteration so drawings are formally approved before work begins.