Basement Staircase Calculator

For a private (within-dwelling) stair, Approved Document K Table 1.1 sets a maximum rise of 220mm, a minimum going of 220mm, a maximum pitch of 42°, and requires 2R+G (twice the rise plus the going) to fall between 550mm and 700mm. Minimum headroom is 2000mm over the pitch line (Diagram 1.3). Note that AD K sets NO minimum width for an ordinary private stair — the only dwelling width figure in AD K is 900mm, and that applies solely to the entrance storey where a severely sloping plot makes a stepped change of level unavoidable. The often-quoted 800mm is a comfort/industry recommendation (and the BS 5395-2 clear width for a primary spiral stair), not a Part K requirement.

The minimum headroom for a private staircase is 2000mm, measured vertically from the pitch line (AD K Diagram 1.3). In basements with restricted headroom, achieving this can be challenging. Solutions include adjusting the stairwell opening position, using winders or a spiral staircase to change direction, or extending the stairwell opening. The reduced-headroom allowance in AD K Diagram 1.4 — 1900mm at the centre of the stair width and 1800mm at the half-width (low-ceiling side) — applies only to loft conversions, not to basements.

Approved Document K sets no minimum width for an ordinary private (within-dwelling) stair, so a 700-750mm flight is not in itself non-compliant. In practice 800-900mm is the usual recommendation for comfort and for moving furniture. The only AD K width figure for a dwelling is 900mm, and that applies solely to the entrance storey on a severely sloping plot (para 1.16). The 1200mm flight width — and the related 1000mm minimum width between handrails — that you may have read about both come from AD K para 1.15, which sits under the heading 'For buildings other than dwellings', so neither applies to a private dwelling stair. (Separately, para 1.34 requires a handrail on both sides of any flight 1000mm or wider, in all buildings including dwellings.) BS 5395-2 sets 800mm clear width for a Category B primary spiral stair.

Yes, spiral staircases are permitted for basements. AD K para 1.28 directs that spiral and helical stairs follow BS 5395-2, which classes a small private spiral serving one room (Category A) at 600mm minimum clear width and a primary private spiral (Category B) at 800mm minimum clear width. Headroom (2000mm over the pitch line) and consistent rise/going still apply. Note that spiral stairs are difficult to move furniture up and down, and a habitable basement still needs the AD B means of escape (an egress window/door or a protected stairway to a final exit).

The 2R+G rule states that twice the riser height plus the going should equal between 550mm and 700mm for a comfortable staircase. For example, a 190mm riser with a 250mm going gives 2(190) + 250 = 630mm — well within the comfortable range. This formula has been used by architects for centuries and is referenced in Approved Document K.