Stairwell opening
made simple.
Find the minimum floor opening you need to fit a Part K compliant staircase. Verify headroom in seconds — before you cut joists.
Five ways to fix a too-small stairwell.
If your existing opening is smaller than the calculated minimum, you have options before deciding to demolish and re-cut joists.
Switch to L-shape
An L-shaped staircase with half landing reduces the required floor opening by ~40%. Fits within most existing stairwells without major joist alteration.
−40% lengthSwitch to U-shape
A U-shaped staircase fits the smallest footprint of any conventional configuration. Two parallel flights with a half landing turn the stair 180°.
−55% lengthReduce stair width
Going from 1000mm to 800mm width saves 200mm of opening width. Still well within Part K minimum of 600mm and comfortable for daily use.
−200mm widthShallower pitch
Adding more risers (e.g. 14 → 16) reduces pitch and headroom obstruction, but extends total run length. Useful where vertical clearance is the limit.
−10° pitchSpiral staircase
A spiral staircase needs only a circular opening (~2000mm dia) — ideal for tight retrofits where joist alteration isn't possible.
~80% smaller footprintRaise the upper floor
Sometimes simplest: raise upper FFL by 100–200mm to gain headroom without changing the opening. Requires structural review.
+200mm headroomUK regulations & further reading.
Authoritative references for staircase compliance and design — official UK Building Regulations and Continox technical resources.
UK Staircase Regulations Guide
Complete guide to UK staircase regulations — Part K headroom, rise & going, pitch limits, balustrade height and BS 5395-1.
Full staircase calculator
Design rise, going, balustrade. Handles straight, L-shape, U-shape and winder configurations.
Modern staircase guide
Comparison of floating cantilever, central spine, and twin stringer — engineering and applications.
What Is a Stairwell Opening & Why Size Matters
A stairwell opening is the rectangular hole in your upper floor that lets a staircase pass through. Its minimum size is determined by stair pitch and the 2000mm headroom rule in UK Building Regulations Approved Document K. Get it wrong and the staircase fails building control — get it right and you save thousands in re-cutting joists.
For a typical UK domestic staircase with 2700mm floor-to-floor rise and a 37° pitch, the minimum opening is approximately 2580mm long by stair-width plus 100mm wide. The exact figure depends on five variables — total rise, ceiling thickness, headroom required, stair pitch, and trimmer beam allowance — which is why this calculator exists.
UK Building Regulations Part K requires a minimum 2000mm headroom over the entire stair flight, measured vertically from the pitch line to any obstruction above. The stairwell opening must extend back along the stairs far enough that this 2000mm clearance is maintained.
How to Calculate Stairwell Opening Size
The minimum stairwell opening length is calculated using a single geometric formula that combines stair pitch with the required headroom. Once you know the pitch angle, the rest is trigonometry.
The Formula
Minimum opening length = (headroom + ceiling thickness) ÷ tan(pitch angle), plus trimmer allowance.
For a UK domestic staircase with 37° pitch, 250mm ceiling and 2000mm headroom required, this gives:
- (2000 + 250) ÷ tan(37°) = 2250 ÷ 0.7536 = 2986mm theoretical minimum
- In practice the opening only needs to extend back to where headroom is first achieved on the pitch line — typically 2400-2700mm for residential stairs
- Add 50-100mm for trimmer beam thickness (timber 47-75mm, steel 100mm+)
Five Inputs You Need
- Floor-to-floor height (total rise) — measured FFL to FFL, typically 2400-3000mm in UK homes
- Stair width — minimum 600mm under Part K, 800-1000mm recommended for daily use
- Use category — private dwelling (max 42° pitch) vs public/commercial (max 38° pitch)
- Ceiling/floor thickness — slab plus floor finishes plus joist depth, typically 220-300mm
- Trimmer beam allowance — added to clear opening to give rough joist span, 50-100mm typical
Minimum Stairwell Sizes by Floor Height
The table below gives approximate minimum stairwell openings for typical UK floor heights, assuming a 1000mm wide stair, 250mm ceiling thickness, 2000mm headroom and 50mm trimmer allowance.
| Floor-to-floor (mm) | Risers | Pitch | Min opening length | Min opening width |
|---|---|---|---|---|
| 2400 | 13 | 37.0° | 2530 mm | 1100 mm |
| 2500 | 14 | 36.4° | 2570 mm | 1100 mm |
| 2600 | 14 | 37.5° | 2570 mm | 1100 mm |
| 2700 | 14 | 38.7° | 2580 mm | 1100 mm |
| 2800 | 15 | 37.8° | 2620 mm | 1100 mm |
| 2900 | 16 | 36.7° | 2680 mm | 1100 mm |
| 3000 | 16 | 37.6° | 2680 mm | 1100 mm |
These figures are indicative for straight flights only. L-shaped staircases need 30-40% less opening length thanks to the half landing breaking the headroom envelope. Use the calculator above to model your specific dimensions.
Part K Headroom Requirements Explained
UK Building Regulations Approved Document K specifies 2000mm minimum clear headroom over the whole flight of stairs. This is measured vertically from the pitch line — the line connecting the front edges of consecutive nosings — to any obstruction above, including the underside of the upper floor or any beam crossing the stair.
For loft conversions, Part K Diagram 1.2 allows a relaxed headroom of 1900mm at the centre of the stair and 1800mm at the side, but only where space is genuinely constrained and the stair serves a single dwelling.
Where Headroom Fails Most Often
- At the top of the stairs, where the stairwell opening edge meets the pitch line — this is the critical point the calculator measures
- Under sloping ceilings in loft conversions, where roof rafters intrude into the headroom envelope
- Below structural beams that run perpendicular to the staircase
- At landing transitions in L-shape and U-shape configurations
For full coverage of all stair regulations including rise, going, balustrade height and pitch limits, see the UK staircase regulations guide.
Trimmer Beam & Structural Allowance
The opening dimensions calculated by this tool are the clear opening — the visible hole between finished surfaces. The actual structural opening cut into the joists must be larger by the thickness of the trimmer beam plus any cladding or boxing.
Typical Trimmer Beam Allowances
- Timber-frame domestic: doubled C24 joist, 47-94mm thick — add 50-100mm to clear opening
- Engineered I-joists: web stiffeners + LVL trimmer, 75-150mm thick — add 100-200mm
- Steel-frame commercial: UB or PFC section, 100-200mm web — add 150-250mm
- Concrete slab: edge of slab is the structural opening — add 50mm for finishes only
Always engage a structural engineer before cutting joists for a stairwell opening. They will specify trimmer size, fixings (joist hangers, bolted plates), and any temporary propping required during the cut. Building control will refuse sign-off without engineer's calculations for openings spanning more than two joists.
Common Stairwell Mistakes in UK Builds
The five most common stairwell sizing errors that cause Part K failures in UK domestic builds:
- Forgetting the trimmer beam — designers calculate the clear opening but cut the structural opening at exactly that size, ending up with a clear opening 50-100mm too short
- Measuring pitch wrong — pitch is measured along the nosing line, not the stringer or the underside of the treads. Using the wrong reference adds 1-2° error
- Ignoring floor finishes — 18mm engineered oak plus 22mm structural ply plus 250mm joists = 290mm total ceiling thickness, often quoted as just "250mm slab"
- Stairs that don't centre on the opening — offsetting the staircase 100mm from one side of the opening can mean the headroom fails before the pitch reaches the calculated point
- Using domestic limits for shared/commercial use — semi-detached with separate flats, HMOs, and offices must use 38° max pitch, not 42° — which adds ~300mm to opening length
Stairwell Sizes by Configuration
Configuration choice has the largest single impact on stairwell opening size — typically larger than any geometric adjustment. The figures below assume 2700mm rise, 1000mm width, 250mm ceiling and 2000mm headroom.
Straight Flight
Single uninterrupted flight from FFL to FFL. Minimum opening: ~2580 × 1100mm. Largest footprint but lowest cost and simplest installation. Standard for new-build homes with generous floor area.
L-Shape with Half Landing
Two flights joined by a half landing at 90°. Minimum opening: ~1550 × 1100mm (40% reduction). Most common in UK Victorian and Edwardian retrofits where the existing stairwell is tight.
U-Shape with Half Landing
Two parallel flights with a half landing turning the stair 180°. Minimum opening: ~1200 × 2200mm (smallest length, double width). Standard in townhouses, terraced properties, and apartment blocks with constrained floorplans.
Winder (90° Turn)
Three tapered treads replace the half landing at the corner. Minimum opening: ~1700 × 1100mm. Compromise between L-shape and pure straight — saves opening size without the structural complexity of a full landing.
Spiral or Helical
Circular stair around a central pole or stringer. Minimum opening: ~2000mm diameter. Smallest footprint of any configuration. Part K limits spirals to secondary access (loft, mezzanine) when serving as the only stair to private accommodation.
For interactive comparison of all five configurations with plan and section views, use the full Continox staircase calculator.