Second staircase requirements and the effect on layout

A second staircase in a tall residential building is not a detail you add at technical design. It changes the core, the corridor length, the net to gross ratio and the unit mix, and on a small floorplate it can take a scheme from viable to marginal. Second staircase tall buildings guidance in England now sets the threshold at 18 metres, following the change announced in 2023 and written into the fire safety guidance in 2024 with transitional arrangements that have since run their course.

Verify the current position before design freeze. The guidance has been amended repeatedly since 2019 and the transitional wording matters as much as the threshold itself.

Why stair provision moved into planning

Stair provision is a building regulations matter. It arrived in planning because the floorplate consequences are so large that a scheme designed around a single core cannot be adjusted later without a new application.

The sequence is worth understanding. In 2023 the government consulted on requiring a second staircase in new residential buildings above 30 metres, then announced an 18 metre threshold instead. The amendment to the fire safety guidance followed, with transitional provisions allowing schemes already in train to proceed on the old basis for a defined period.

Alongside that sits the Building Safety Act 2022 regime, under which buildings of at least 18 metres or at least seven storeys with two or more residential units are higher-risk buildings subject to the three gateways. Gateway One operates at planning application stage through Article 9A of the Development Management Procedure Order 2015, which requires a fire statement for relevant buildings and makes the Health and Safety Executive a statutory consultee.

So the planning application for a tall residential building now carries a fire document, consults a regulator and depends on a core arrangement fixed by guidance outside the planning system. Our note on which mixed use buildings need an Article 9A fire statement covers the trigger in more detail.

Height thresholds and their verification

Height is measured to the floor surface of the top storey, excluding any storey that is a roof-top machinery or plant area, measured from ground level on the lowest side of the building. That definition is what produces arguments on sloping sites.

Three practical points follow:

1. On a site falling 1.5 metres across the footprint, the measurement is taken from the lower ground level, so a building that reads as 17.5 metres on the high side may exceed 18 metres on the low side.
2. Plant enclosures and lift overruns at roof level are generally excluded, but a roof terrace with habitable use above the top storey needs checking.
3. The seven storey limb of the higher-risk building definition catches buildings below 18 metres with low floor to floor heights, which is common in conversions.

Get the measured height certified by the design team and stated on the drawings. A single section with the measurement line and the datum removes a whole category of consultee queries.

Check each figure against the current Approved Document B and the current Order, because all three have been amended.

Net to gross effect of a second core

A second stair and its lobby consume floor area on every storey. On a large floorplate of 1,200 m² the proportional loss is modest. On a 450 m² floorplate serving eight flats it is severe, because the stair, lobby and the extra corridor length needed to reach it eat into the lettable area at a fixed cost per floor regardless of building size.

The secondary effect is often worse than the direct one. A second stair positioned to give acceptable travel distances frequently pushes the corridor into a dog-leg, which creates inner corners, reduces the number of dual aspect units and generates single aspect units facing the worst elevation. That interacts with daylight, and the units created in those corners are the ones that fail an internal daylight check. Our note on piling methods beside sensitive neighbours covers a related coordination problem where the structural solution drives the planning drawings.

Run the arithmetic before you commit to a massing. If a 450 m² floorplate with two cores cannot deliver the unit count the appraisal assumed, the answers are a larger footprint, fewer storeys with a different structural approach, or a different site.

Unit mix consequences on small floorplates

The units that disappear first are the smallest ones, because they are the ones that fitted into the leftover corners. What tends to survive is a mix weighted towards two bedroom units in the middle of the plan.

That collides with local plan mix policy. Where the authority requires a proportion of three bedroom family units, delivering them on a two core floorplate with the required daylight and dual aspect expectations gets hard, and the resolution is usually fewer, larger units overall.

Deal with it in the design and access statement rather than leaving the officer to spot the shortfall. A clear explanation that the mix follows from the stair requirement, with the tested alternatives shown, is a much better position than a mix that looks like a commercial choice.

Phasing and revisions to consented schemes

Schemes consented before the guidance changed, and not built out, are the difficult cases. A single-stair consent that is now being built to a different standard needs a planning amendment, and the extent of the change usually takes it beyond a non-material amendment under section 96A.

The realistic routes are a section 73 application to vary the approved plans where the change is not fundamental, or a fresh full application where it is. A second core typically alters elevations, unit numbers and the internal layout, so expect the authority to require the fuller route.

Watch the knock-on obligations. A section 73 permission is a new permission, and the authority may seek an updated section 106 agreement, revised biodiversity net gain figures and current policy compliance on energy and overheating. The arithmetic on a reduced unit count with unchanged obligations is sometimes the reason a consented scheme stops.

An illustrative example

Take an illustrative case: a consented eight storey block, 24.5 metres to the top floor, with a single core on a 520 m² floorplate delivering 56 flats. Redesign with a second protected stair and lobby removes about 38 m² per floor and lengthens the corridor, reducing the floorplate's usable area enough to lose eight flats. The revised mix drops from 14 one bedroom units to six, and two of the three bedroom units become single aspect. The scheme proceeds as a fresh application with a revised fire statement and an updated obligation. These figures are illustrative and not from a real scheme.

Interaction with the fire statement

The Fire Statement submitted at Gateway One should describe the stair arrangement, the evacuation strategy, firefighting access and the water supply, and it must be prepared by someone with the necessary fire engineering competence. This is one of the report types where a competent professional is legally required, and an AI-assisted draft is a starting point for that professional rather than a substitute.

The deeper strategy work, including smoke control and the evacuation approach for the chosen stair arrangement, belongs in a Fire Safety Strategy prepared to the relevant British Standard. Again, that is a fire engineering document.

Coordinate the two with the planning drawings. The most common failure is a fire statement describing a stair arrangement that the submitted floor plans do not show, which the HSE picks up and which costs weeks.

Checking the current position before design freeze

Before you fix the massing, confirm four things in writing: the measured height to the relevant datum, whether the building is higher-risk under the 2022 Act, whether the current Approved Document B requires a second stair at that height, and whether any transitional provision could apply to your programme.

Then record the answers in the design report with dates. A design decision taken against the guidance as it stood on a stated date is defensible. A design decision taken against a summary somebody read a year earlier is not.

Where structural matters follow from the core arrangement, our note on structural appraisal for new homes above a shop unit covers the equivalent coordination problem at smaller scale.

Enter your postcode on PlanScope to see which reports a tall residential scheme is likely to need.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required. This is general guidance, not legal advice; check the position with your local planning authority, building control body or a qualified professional before relying on it.