Summary

The staircase to a loft conversion is one of the most technically and practically constrained elements of the project. Unlike an extension stair (which can usually be positioned freely), a loft stair must fit within the existing floor plan of the floor below, often carving space from a bedroom, landing, or bathroom that the occupants want to keep. This spatial constraint means that building a loft stair that fully satisfies Approved Document K's geometric requirements — without compromise to the floor below — is the central design challenge of most loft conversion projects.

Approved Document K sets out the rules for stairs in England and Wales. It distinguishes between "common" stairs (used by more than one household) and "private" stairs (within a single dwelling). Loft conversion stairs are always private stairs, which allows slightly more flexibility than common stairs — in particular, the permitted use of alternating tread stairs as a space-saving measure. However, this flexibility is often misunderstood: alternating tread stairs are a concession of last resort, not a first choice, and must only be used where a conventional stair genuinely cannot be accommodated.

Fire safety requirements interact with stair design at a key point: where a loft conversion creates a three-storey dwelling (ground floor + first floor + loft), the stair must form part of a protected escape route. This means fire-resisting construction to the stair enclosure and a self-closing fire door (FD30S) at the bottom of the loft stair. On a two-storey house with a loft room accessed off the first-floor landing, this requirement is typically straightforward — but the detailing matters.

Key Facts

  • Maximum rise (private stair) — 220 mm per step
  • Minimum going (private stair) — 220 mm per step (measured horizontally from nosing to nosing)
  • 2R + G formula — should fall between 550 mm and 700 mm; this is a comfort check, not an absolute requirement in AD K for private stairs
  • Minimum headroom (private stair) — 2000 mm measured vertically from the pitch line at the centre of the stair
  • Loft stair headroom concession — 1800 mm measured vertically at the side of the stair (i.e. under the slope of the ceiling soffit at the edge), where the full 2000 mm cannot be achieved centrally
  • Handrail height — 900 mm minimum to 1000 mm maximum, measured vertically from the pitch line of the stair
  • Landing handrail height — 900 mm minimum on landings within the dwelling
  • Balustrading sphere rule — balustrades and guarding must prevent the passage of a 100 mm diameter sphere (not 100 mm gap — the sphere test is applied to the open space, including diagonal gaps)
  • Alternating tread stair — minimum going (wider tread) — 220 mm (measured at the wider portion of the tread)
  • Alternating tread stair — maximum pitch — 42°
  • Alternating tread stair — handrail both sides — required; handrails on both sides are mandatory for alternating tread stairs
  • Open risers — permitted on private stairs if a 100 mm sphere cannot pass between treads
  • Fire door at loft stair base — FD30S (self-closing fire door, 30-minute fire resistance) required in three-storey dwellings
  • Minimum stair width — no absolute minimum in AD K for private stairs; 800 mm is the practical minimum for furniture movement
  • Maximum pitch (standard private stair) — 42° (implied by max rise 220 mm and min going 220 mm)
  • Winder treads — permitted; minimum going at narrow end must not be less than 50 mm; going at 270 mm from narrow end must comply with minimum

Quick Reference Table

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Parameter Standard Private Stair Alternating Tread (Loft)
Max rise 220 mm 220 mm
Min going 220 mm 220 mm (wider tread)
Max pitch 42° 42°
Min headroom (centre) 2000 mm 2000 mm (1800 mm at side)
Handrail height 900–1000 mm 900–1000 mm (both sides mandatory)
Handrail — both sides required? One side minimum if width ≥1000 mm Both sides mandatory
Balustrading sphere rule 100 mm 100 mm
When permitted Always Only where standard stair cannot fit
Fire door at base FD30S (3-storey) FD30S (3-storey)
Open risers Permitted (sphere rule) Not recommended

Detailed Guidance

Rise and Going — Getting the Geometry Right

The step geometry is the starting point for any stair design. Rise is the vertical height of each step. Going is the horizontal distance from one nosing to the next. For a private loft stair, the upper limits are 220 mm rise and no lower limit on going above 220 mm — but the 2R + G formula (which should give 550–700 mm) is a useful comfort check even where not strictly mandated.

In practice, the floor-to-floor height (the total rise) and the available plan length (which determines the total going) are fixed by the building. The number of steps is then derived as: total rise ÷ desired rise per step, rounded to the nearest whole number. This gives the actual rise per step, which must be checked against the 220 mm maximum. Each step must be equal — a variation of more than 5 mm between any two steps is a trip hazard and will fail Building Control.

Example calculation:

Total floor-to-floor height: 2700 mm Target rise: 200 mm Number of steps: 2700 ÷ 200 = 13.5 → round to 14 Actual rise: 2700 ÷ 14 = 192.9 mm (acceptable, under 220 mm) Required plan length: 14 steps × 220 mm going minimum = 3080 mm

If 3080 mm of plan length is not available within the floor below, the stair design must be modified — either with winders, a half-landing turn, or (as a last resort) an alternating tread stair.

Headroom Requirements

The 2000 mm headroom requirement for private stairs is measured vertically from the pitch line of the stair — an imaginary line connecting the nosings of each step. This means headroom is not measured from the floor at any given point, but from the step nosings themselves. The practical implication is that a person standing on the step with their feet on the nosing has 2000 mm to any obstruction above.

For a loft stair emerging through the loft floor, the underside of the new loft floor joists typically forms the soffit above the upper portion of the stair. The headroom at the top of the stair — where the stair passes through the floor — is the point most likely to fail, because the floor buildup above is often thicker than expected (200–250 mm finished floor plus joist depth).

The concession for loft stairs (1800 mm at the side where a sloping ceiling reduces headroom) is valuable but limited. It applies where the ceiling slopes as a result of the roof construction — for example, where the stair emerges into the loft room and the last few steps are under the slope of the roof. In this zone, 1800 mm is acceptable at the side of the stair (under the lowest point of the slope), provided 2000 mm is maintained at the centre. This is not a blanket reduction — the 2000 mm centreline rule applies throughout the rest of the stair.

Headroom check sequence:

1. Mark pitch line on stair section drawing (line joining nosings)
2. Measure perpendicular vertical to nearest obstruction above at each step
3. Is measurement ≥2000 mm at all points on centreline?
   YES → Compliant. Done.
   NO → Is the low point within the loft room, under a sloping roof?
     YES → Is measurement ≥1800 mm at side and ≥2000 mm at centre?
       YES → Loft concession applies. Compliant.
       NO → Redesign required: raise floor opening, lower stair, or reposition.
     NO → Redesign required. Cannot use loft concession elsewhere.

Alternating Tread Stairs

Alternating tread stairs (also called paddle stairs or space-saver stairs) are specifically recognised by Approved Document K as a permitted solution for loft conversions where a standard stair cannot fit. They work by alternating the position of each tread so that left-foot and right-foot steps are offset — effectively combining the function of two steps into one plan footprint.

The requirements for alternating tread stairs under AD K are:

  • Maximum pitch: 42°
  • Minimum going of the wider portion of each tread: 220 mm
  • Handrails on both sides (mandatory — this is an absolute requirement, not a recommendation)
  • Must only be used where a standard stair cannot be accommodated

The last point is important. Building Control officers will expect justification that a standard stair has been explored and cannot fit. Installing an alternating tread stair as the default choice because it saves money or simplifies the floor plan is not an acceptable reason. "We looked at a standard stair and the plan length required (approximately 3 m) would take this entire bedroom — here is the dimensioned floor plan showing this" is the kind of justification that will satisfy the BCO.

Alternating tread stairs are not suitable as the primary access for elderly users, young children, or anyone with mobility issues. For a family home, this should be discussed with the client before specifying.

Handrails and Balustrading

Handrails must be at 900 mm–1000 mm height, measured vertically from the pitch line of the stair. On landings, the handrail (or guarding) height is 900 mm minimum measured from the landing surface.

On stairs over 1000 mm wide, handrails must be provided on both sides. On stairs under 1000 mm wide (which includes most domestic loft stairs), one handrail is the minimum — though two is better practice and is mandatory for alternating tread stairs.

Handrail profile matters: AD K requires handrails to be grippable — a circular section (between 32 mm and 50 mm diameter) or an oval section (between 50 mm wide and 70 mm wide, 32 mm deep, with rounded edges) is required. A flat-top balustrade rail that cannot be gripped does not comply as a handrail, even if it is at the right height.

The 100 mm sphere rule applies to any guarding (balustrading) required where a fall of more than 600 mm is possible — this includes the stair balustrade and the guarding around the loft floor opening. The test is that a sphere of 100 mm diameter must not be able to pass through any opening in the guarding. This applies in all directions — including diagonally. Spindles at 100 mm on centre (centre-to-centre) will still pass the test for a vertical gap (open width is less than 100 mm), but if the spindles are inclined or there is a diagonal gap from spindle top to handrail, the sphere check must be applied to that diagonal too.

Glass guarding around the stair void is popular for aesthetic reasons. Any glass used as guarding must be toughened safety glass (to BS EN 12150 [verify]) or laminated safety glass (to BS EN ISO 12543 [verify]).

Fire Door Requirement at Loft Stair Base

When a loft conversion creates or extends a three-storey dwelling, the stair to the loft forms part of the protected escape route. This requires:

  1. A self-closing fire door (FD30S — 30-minute fire resistance, self-closing device fitted) at the foot of the loft stair, opening onto the first-floor landing
  2. Fire-resisting construction (30-minute minimum) to the walls, floor, and soffit of the stair enclosure

The fire door at the stair foot must be FD30S, not just FD30. The "S" indicates smoke-sealing — intumescent strips and cold smoke seals are required around the door frame. The door must be self-closing (a quality door closer, not a spring).

The requirement applies to conversions that create a three-storey dwelling. For a bungalow loft conversion, the same logic applies if the conversion creates a two-storey dwelling from a single-storey building — Approved Document B will require the stair to be a protected stairway. Always check the full Building Regs fire safety requirements against the specific building type.

Where the first floor landing already has fire-resisting construction (common in many post-1980 houses which were built with 30-minute protection between floors), the existing doors may need upgrading to FD30S if they open onto the stair enclosure.

Positioning the Stair Within the Floor Plan

The stair must connect the loft floor to the floor below via a hole (well) cut through the floor structure. The position of this well determines what space is lost from the floor below and governs much of the room layout above.

Common positions and their implications:

Position Typical room lost below Impact
Landing (straight up) Landing width Often good — no bedroom lost
Through smallest bedroom Small bedroom lost or reduced Common compromise
Through bathroom (unusual) Bathroom lost Rarely acceptable
Corridor (through hall) Hallway headroom Can work on deep plans

The stair well through the floor structure must be properly trimmed (doubled trimmer joists around the opening, same as a dormer opening), and the structural implications assessed by the SE. See loft conversion floor structure for detail on trimmer sizing.

The top of the stair (the loft landing) must have a clear landing area of at least the stair width × the stair width — typically 800×800 mm minimum. The landing must be level and must not be partially obstructed by a door swing. Building Control officers check this carefully.

Frequently Asked Questions

Can I use a spiral staircase for a loft conversion?

Spiral stairs are not specifically prohibited but are extremely difficult to meet Part K requirements — particularly the going measurement (220 mm minimum at the 270 mm point from the narrow edge of each tread) and the headroom requirement. A very large-diameter spiral stair (1800 mm diameter+) can comply, but it takes more plan area than a straight stair. Most Building Control officers will want to see the geometry worked out in detail before approving. Spiral stairs are generally not recommended for loft conversions.

Does the fire door at the bottom of the loft stair have to be new?

Not necessarily, but it must meet the FD30S specification — 30-minute fire resistance plus smoke seals and self-closing device. An existing door can be upgraded with an intumescent smoke seal kit (fitted into the frame rebate) and a self-closer, but only if the door leaf itself is FD30. Most domestic internal doors are not FD30 unless explicitly specified as such. In practice, installing a new FD30S door is usually more reliable than attempting to upgrade an existing one.

What if my loft headroom is only 1.8 m at the top of the stair?

This is the loft concession — 1800 mm is acceptable at the side of the stair (under a sloping soffit) where the full 2000 mm cannot be achieved due to the roof construction. This only applies at the transition point where the stair emerges into the loft room under the slope. If the 1800 mm restriction extends down the full run of the stair, that is not what the concession covers — the stair will need redesigning.

Do both sides of the loft stair need a handrail?

On a stair under 1000 mm wide, one handrail is the minimum under AD K. However, if the stair is an alternating tread (paddle stair), both sides are mandatory regardless of width. In practice, two handrails are better and safer on any loft stair — the stair is typically steep and often used at night.

How far from the top step can the stair void be left open before guarding is required?

Guarding is required at any edge where there is a drop of more than 600 mm. At the top of the stair, the opening in the loft floor must be guarded — typically with a balustrade rail, a hinged hatch cover (if the opening is small), or a retractable gate. An unguarded open stair void in a loft is a hazard and will not pass Building Control.

Regulations & Standards

  • Approved Document K (2013 edition) — Protection from Falling, Collision and Impact; primary reference for stair geometry, handrails, and guarding

  • Building Regulations 2010 (as amended) — Regulation 7 (materials and workmanship), Schedule 1 (functional requirements)

  • Approved Document B Volume 1 (2019) — Fire Safety in Dwellings; protected stairways, fire doors (FD30S), fire-resisting construction

  • BS 8300:2018 — Design of an accessible and inclusive built environment; guidance on stair accessibility (relevant where occupant has mobility needs)

  • BS EN 12150 [verify] — thermally toughened soda lime silicate safety glass; specification for glass used in guarding

  • BS EN ISO 12543 [verify] — laminated glass specification; alternative to toughened glass for guarding

  • BS 5395-1:2010 — Stairs — code of practice for the design of stairs with straight flights and winders; aligns with AD K geometric requirements

  • Approved Document K (2013) — GOV.UK; primary regulatory reference

  • Approved Document B Volume 1 (2019) — GOV.UK; fire door and protected stairway requirements

  • LABC — Loft Conversion Guidance — Local Authority Building Control technical guidance [verify specific document]

  • Staircase Manufacturers Association — Technical Notes — industry reference for stair geometry and specification [verify]

  • loft conversion building regs overview — all Building Regulations parts relevant to loft conversions

  • loft conversion fire escape — fire escape requirements including protected stairways

  • loft conversion floor structure — floor structure strengthening and trimmer sizing for stair well

  • staircase regs — broader staircase regulations reference