Second staircase provision in tall residential design

Second staircase provision at 18 metres for residential buildings is a Building Regulations requirement delivered through Approved Document B, not a planning policy. It still decides planning submissions, because a second stair changes the core, the floor plate, the massing and often the elevations, and none of those can be revisited after permission without a fresh application. So the decision has to be taken at feasibility stage, on a threshold measured the same way the Gateway One duty is measured.

Architects working on point blocks have felt this most sharply, because a small floor plate carries a second core very badly.

Where the expectation comes from and its status

The sequence matters because it explains why the guidance moved. Government consulted on a second staircase requirement for new residential buildings with a threshold of 30 m, then announced in October 2023 that the threshold would instead be 18 m, aligning it with the threshold already used for the higher-risk regime and for Planning Gateway One.

Delivery is through an amendment to Approved Document B rather than through planning policy or primary legislation. The amendment was published in 2024 with an implementation date announced as 30 September 2026 and with transitional arrangements for schemes already sufficiently progressed.

Verify the current status, the implementation date and the exact transitional wording against the published Approved Document B before relying on any of it. Those three things decide whether a particular scheme is caught, and they have moved once already.

Alongside the staircase question, government has signalled requirements relating to evacuation lifts in new residential buildings above the same threshold, and sprinkler provision has been required in blocks over 11 m since the 2020 amendment. Treat the core design as a package rather than as a stair count.

The height threshold and how it is measured

Height is measured from ground level to the floor surface of the top storey, excluding any storey that consists exclusively of plant rooms. That is the same definition used for a relevant building at Gateway One, which is helpful: one measurement answers both questions.

Three measurement traps recur.

Sloping ground. The measurement is taken from ground level, and on a site with a 2 m fall the elevation you measure from changes the answer. Measure from the lowest adjoining ground level unless the published guidance directs otherwise, and show the measurement on a section.

Roof-top plant. A plant enclosure above the top occupied storey does not count, but a storey containing plant and anything else does. A rooftop plant room with a maintenance store or a lift motor room plus a communal space is not a plant-only storey.

Mezzanines and duplexes. The top storey is the top storey, and an upper level of a duplex flat is a storey. Schemes designed to sit just under the line frequently cross it on the upper level of the penthouse.

A scheme coming out at 17.6 m is not a scheme below the threshold. It is a scheme with no tolerance, and any change in ground level treatment, floor zone depth or slab thickness during detail design can push it over.

Plan consequences of a second stair

A second stair is not one shaft. It is a protected stair enclosure, its own lobby, its own smoke control provision, and the structural and service implications of carrying both through every floor plate.

The plan consequences run in this order.

1. Core area grows on every storey, including the ground floor where the second stair has to discharge independently to a final exit.
2. Each stair needs its own protected route to a final exit, which on a constrained site often means a protected corridor at ground level running to a second street frontage.
3. Smoke control for two stairs is a different system from smoke control for one, with implications for shaft size and for the facade where natural ventilation is used.
4. Risers compete with the second stair for the same part of the plan, and moving them is structurally expensive.
5. Corridor geometry changes, because two stairs allow escape in two directions and that changes the permissible travel distances and the corridor length.
6. Lift provision has to coexist with both stairs and with an evacuation lift where one is required.

Point two is the one that catches urban infill schemes. A single-aspect site with one street frontage and a party wall on both flanks may have nowhere for a second independent final exit to go, and the answer then is a design change rather than a detail.

Effect on unit mix and net to gross

Net to gross is where the commercial argument happens. For blocks of flats a net to gross ratio somewhere in the high seventies to mid eighties per cent is the field convention, and a second stair pushes the figure down.

The loss is proportionally brutal on small floor plates. A second core on a 1,200 m² plate is absorbed. The same core on a 400 m² plate takes a share of the floor that makes the storey uneconomic, which is why the 18 m threshold has pushed designers towards larger, lower buildings rather than taller slim ones.

Two compensating effects are worth modelling rather than assuming away. Two stairs allow escape in two directions, which lengthens permissible travel distances and can therefore support more units per core than a single-stair arrangement at the same floor area. And a second stair removes the single-stair constraints that limit the number of dwellings per storey in some single-stair guidance, which can offset part of the core loss.

So run the comparison properly: units per storey and net saleable area per storey for the single-stair option, the two-stair option at the same envelope, and the two-stair option at an enlarged envelope. The third usually wins, and it is a planning conversation about massing rather than a fire conversation.

Transitional arrangements and the as-at position

Transitional provisions in the Building Regulations normally turn on the date of the building control application and on whether work has progressed sufficiently by a stated date. The precise test and the relevant dates sit in the amendment itself.

For a planning submission the practical question is different. Planning permission does not fix the applicable Building Regulations standard, so a scheme consented on a single-stair layout that then has to be built with two stairs needs a planning variation. That is an expensive and visible place to discover a transitional issue.

The defensible approach is to design for the requirement where the building is above 18 m, whatever the transitional position appears to be, unless the programme is certain enough to rely on it. And where you do rely on a transitional provision, record the reliance in the project file with the date and the version of the guidance consulted, because the person asking the question in two years will not be you.

Interaction with the Gateway One submission

The Regulator reviews the fire statement at planning stage and the stair strategy is central to what it asks about. A statement describing a single-stair arrangement in a building above 18 m needs to explain the basis on which that is proposed, including any transitional reliance, rather than presenting it without comment.

Set out in the statement the measured height with the section that supports it, the number of stairs and their arrangement, the final exit arrangement for each, the smoke control principle, and the evacuation strategy including any evacuation lift. The detailed engineering belongs in the fire safety strategy prepared to BS 9991 or BS 9999, and the planning-stage document is the fire statement, which is a report type where a professional with fire engineering competence is legally required.

Where the structure is mass timber, the stair enclosure question becomes more involved again, and the issues set out in the discussion of mass timber schemes and the questions planners ask interact directly with the core design.

Designing a core that works either way

For schemes near the threshold, design the core so a second stair can be added without redrawing the building.

Reserve a zone of the plan adjacent to the primary core, aligned with the structural grid, dimensioned to take a stair and a lobby at the required widths. Position risers so that they are not in that zone. Plan the ground floor so a second protected route to an independent final exit is physically available, even if it is initially used as something else. And keep the facade module able to accommodate smoke ventilation openings in the reserved zone.

That reservation costs area in the interim, which is the honest trade. The alternative, on a scheme that crosses the threshold during detail design, is a new planning application.

One further point about basements, which often share the core: escape from a basement level and its interaction with other constraints is its own problem, and the combined issues described in the note on basement light wells, escape and flood risk together frequently determine where the second stair can discharge.

This is general guidance, not legal advice; check the position against the current Approved Document B and with a qualified fire engineer before relying on it. 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. You can enter the site postcode to see which fire and structural reports a tall residential scheme is likely to need.