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Loft Staircase Space Calculator — Floor Opening & Area Lost
Calculate the floor opening size and area lost on the floor below for different loft staircase types: straight, winder, spiral and space-saver.
Finished floor to finished floor
Landing at top/bottom of stairs
Height per step — AD K Table 1.1 private stair: 150-220mm; alternating-tread/space-saver loft stairs are also capped at 220mm max rise (AD K paras 1.29-1.30, Diagram 1.10)
Tread depth — AD K Table 1.1: private stair 220-300mm; utility stair allows 220mm minimum going; alternating-tread loft stairs 220mm min (AD K Diagram 1.10)
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How We Calculate This
This calculator determines the floor opening size and area impact for different loft staircase types.
The formula
Opening Area = Opening Length × Opening Width
Risers = ceil(Floor-to-Floor Height ÷ Rise Height), and Actual Rise = Floor-to-Floor Height ÷ Risers (the rise is shared equally across every step).
Pitch = atan(Actual Rise ÷ Going) in degrees. Approved Document K defines the pitch as the angle of the pitch line — the line joining the nosings — so it is measured from one individual rise over one individual going, not the overall rise over the whole run.
The tool also checks the AD K usability rule 2 × Rise + Going = 550–700mm and flags a private-stair pitch over the 42° limit.
Typical opening sizes
- Straight flight: 2.6m × 0.9m (2.34 m²)
- Winder/L-shape: 1.8m × 1.8m (3.24 m²)
- Spiral: 1.6m × 1.6m well opening (~2.56 m²)
- Space-saver: 2.2m × 0.6m (1.32 m²)
The landing and total area-impact figures are indicative estimates to help you plan, not standard-derived values. AD K headroom (2.0m, relaxed to 1.9m at the centre of the flight and 1.8m at the edge under a sloping loft roof) is not assessed here and must be checked on site.
Frequently Asked Questions
A straight flight needs about 2.6m × 0.9m floor opening (2.34m²). A winder/L-shape needs 1.8m × 1.8m (3.24m²) but fits into a corner. A spiral staircase needs the smallest opening of the conventional types at 1.6m × 1.6m (2.56m²) but may not comply with Part K for a habitable loft room. A space-saver (alternating tread) stair has the smallest footprint of all at 2.2m × 0.6m (1.32m²) but is only permitted to serve a single habitable room.
Space-saver (alternating tread or paddle) stairs are only acceptable for a single habitable room in the loft. If you have two or more rooms (or a bathroom), you need a standard staircase complying with Part K. Building control will advise on your specific situation.
Part K allows a maximum pitch of 42° for a standard staircase. Loft conversions may use an alternating tread stair at steeper pitches if only serving one room. The rise should be 150-220mm and the going 220-300mm, with 2×Rise + Going = 550-700mm.
The ideal position is above the existing staircase, rising in the same direction to minimise disruption and structural alterations. Alternatively, over a hallway or large bedroom. The staircase must discharge onto a landing with a protected escape route — usually the existing landing.
Yes. The loft staircase must be within a protected enclosure with fire doorsets to every habitable room opening onto the stairway, and the existing staircase below must also be enclosed to create a protected escape route from the loft to the final exit door. This is a Part B (Approved Document B Volume 1) requirement. In a single-family house those doorsets need only E 20 (para 2.21b and Table C1 item 11b), with no cold smoke seal, and existing doors are upgraded where necessary. FD20 doorsets are no longer certified as products, so an FD30 door is the practical way to meet E 20. Under AD B, only a fire door to an attached or integral garage must have a self-closing device: closers are not mandatory on the other dwellinghouse fire doors, though some building control bodies recommend them.
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.