Single stair buildings and the planning consequences

Stair provision became a planning question the moment the second staircase requirement for tall residential buildings settled at 18 metres. Above that height a single stair building no longer works in England, and the consequences run straight through the things a planning permission fixes: footprint, massing, unit mix, dual aspect, net to gross and, on schemes where the numbers were tight, viability.

Architects who treat this as a building regulations problem to solve after permission find out late that the core they need does not fit inside the consented envelope.

Why stair provision became a planning issue

The requirement itself sits in the Building Regulations and Approved Document B rather than in planning policy. What makes it a planning matter is that a second stair is not a detail. It occupies floor area on every storey, it changes the corridor layout, and on a slender plot it can make the consented footprint undeliverable.

An amendment to Approved Document B introduced the second stair requirement for new residential buildings above 18 metres, together with evacuation lift provision, subject to transitional arrangements for work already under way. Check the current text of the Approved Document and the precise wording of the transitional provisions, because the dates and the scope of the transition are the part most often misremembered.

Alongside that, the fire statement route at Planning Gateway One brings the Health and Safety Executive into the planning process as a statutory consultee for relevant buildings, which means the fire strategy is being read by a regulator at the application stage rather than after it.

Effect of a second stair on net to gross

The arithmetic is unforgiving because it repeats on every floor. A second stair core is not just the stair. It is the stair enclosure, the lobby, the additional shaft walls and the loss of the corridor length that the stair position consumes.

Multiply that area by the number of storeys and compare it with the gross internal area of the building. On a compact tower with a small floorplate the proportion lost is materially higher than on a large floorplate, because the core is close to a fixed size while the floorplate is not. Any percentage quoted for one scheme tells you nothing about another, so do the calculation on your own drawings rather than reaching for a rule of thumb.

Three secondary effects follow:

1. Unit mix shifts, because the residual area after two cores often suits fewer, larger units or a different bedroom split.
2. Dual aspect provision becomes harder, since a second core interrupts the perimeter and can strand single aspect north-facing units.
3. Amenity and circulation standards, including corridor widths and the number of units per core, come under pressure at the same time.

Where a design review panel or a London borough applies dual aspect and single aspect policies, those secondary effects become policy issues rather than design preferences.

Core position and its consequences for layout

There are essentially three arrangements, and the choice is made early.

Two stairs within one enlarged core gives a compact plan and a simple structure, but it concentrates the core and can leave long single-direction corridors.

Two separated cores at opposite ends of the floorplate gives better escape geography and often better dual aspect, at the cost of more shaft area and a harder structural transfer at podium level.

A scissor arrangement, where two stairs interlock within one shaft, saves area but is unforgiving to build and needs careful treatment of smoke control and lobbying.

Each has a different effect on the elevation, because cores generate blank or lightly fenestrated bays, and on the ground floor, where two stairs need two discharge points to the outside.

Evacuation lift provision alongside stairs

Evacuation lifts are the other half of the change, and they are not simply a lift with a sign on it. They need a protected lobby, a secondary power supply, a communications system and a management procedure that someone owns after handover.

In planning terms the visible consequences are the additional shaft, the plant space and the overrun, which can affect the roof profile and the height stated on the approved drawings. On a scheme where height was negotiated to the nearest half metre, that matters.

The London Plan already carried fire safety and inclusive access policies covering evacuation lifts and fire strategies, so in London the requirement often lands on a design that had partly accounted for it. Elsewhere it may not have.

Schemes designed before the change

The awkward cases are permissions granted against a single stair design that have not yet started, or that have started under transitional arrangements but face a redesign for commercial reasons.

Work through the position in this order:

1. Establish the building height by the measurement the Approved Document uses, not by the height on the planning drawings, since they are measured differently.
2. Confirm whether the transitional arrangements apply to the specific building, on the facts and dates.
3. Test whether a compliant second stair fits inside the consented envelope without changing the external appearance.
4. If it does not, identify which approved drawings change and by how much.
5. Decide the planning route before the contractor prices the revision.

A Fire Statement at Gateway One must be prepared by someone with the necessary fire-engineering competence under the Building Safety Act 2022 framework, and a Fire Safety Strategy to the relevant British Standard is fire-engineering work where a licensed or competent professional is legally required. An AI-assisted draft of either is a starting point for that professional rather than a substitute.

Amending a permission to add a stair

Which route applies depends on what actually changes on the approved drawings.

The drawings condition is the one to read first. Where a permission is granted subject to a condition requiring development to be carried out in accordance with listed approved drawings, any departure from those drawings needs an application even if the change is internal.

Be careful with piecemeal amendments on a phased site. Where a series of separate permissions is used to alter part of a development, the interaction between them has been tested in the courts and the result is not always what the applicant assumed. Take advice before relying on a standalone permission over part of a consented scheme.

Schemes adding storeys to existing blocks raise the same questions from the other direction, and the evidence expected is set out in our note on airspace development above flats.

Viability consequences worth stating

Lost saleable area on every floor changes the residual value, and on schemes where affordable housing was agreed at the edge of viability the effect is real rather than rhetorical.

If you intend to revisit an obligation, the argument has to be evidenced in a Viability Assessment prepared to the recognised professional methodology, with the area loss measured from drawings rather than estimated, and with the benchmark land value unchanged from the original appraisal unless there is a reason to move it. Councils expect a suitably qualified author and professional review is strongly advised, and many will publish the assessment.

Say plainly that the change is regulatory rather than elective. A scheme redesigned to meet a mandatory fire safety requirement is a different conversation from one redesigned to improve returns, and officers respond to that distinction.

Illustrative example: a 21-storey residential building on a constrained urban plot, consented with a single central core. The second stair fits only by moving the core off-centre, which pushes two units per floor from dual to single aspect and reduces the count by one unit on 19 floors. The route chosen is a section 73 application with revised floor plans and a short addendum to the design statement, submitted before the contractor prices the frame rather than after.

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

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