Second staircase requirements and planning applications

A second staircase requirement changes a planning application in ways that go well beyond the fire strategy. Adding a stair and its lobby to every floor of a residential building costs floor area on each storey, removes units, reduces the net to gross ratio and sometimes forces the footprint or the height to change. If the building already has permission, it also raises the question of whether the change can be handled by a variation or needs a fresh application.

The threshold is 18 metres. Government confirmed that figure in 2023, in preference to the 30 metres originally consulted on, and Approved Document B was amended to require a second staircase in new residential buildings above that height, with a lead-in period and transitional provisions.

Where the requirement comes from

The requirement sits in building regulations guidance rather than in planning policy, which is why it catches planning teams out. It applies through Approved Document B, and it interacts with the wider framework created by the Building Safety Act 2022, including the Gateway process and the role of the Building Safety Regulator.

Planning becomes involved because the consequences are physical. A planning permission controls layout, footprint, height and external appearance, and a second staircase affects all four. So a change driven by building regulations has to be reflected in the planning permission before it can be built.

Alongside the national position, the Greater London Authority adopted an interim expectation ahead of the national change, so London applicants should check current Greater London Authority guidance as well as the Approved Document.

As at October 2026, the commencement and transitional arrangements are the detail that matters most for programming. Verify both on gov.uk before fixing a design or a submission date, because a scheme that falls either side of the transitional cut-off is a different project.

Height thresholds and how height is measured

The 18 metre figure is measured to the floor surface of the top storey, excluding any storey that consists only of plant rooms. That definition is the one to use when testing a scheme, and it is not the same as the overall height of the building to its parapet or to the top of a lift overrun.

The practical consequence is that floor-to-floor dimension decides the outcome. A six storey building with generous floor zones can exceed 18 metres while a seven storey one with tighter zones does not, and a late increase in the services zone can push a scheme over the line after the planning drawings are complete.

Three checks are worth doing at concept stage:

1. Calculate the top storey floor level above the lowest adjacent ground level, using the definition in the Approved Document rather than a general height.
2. Record the margin in millimetres, not as a comfortable or tight description.
3. Test what happens if the floor zone grows by 100 millimetres per storey.

Where a scheme sits close to the threshold, the evidence needed for a building just below the threshold deserves attention in its own right, because the consultee will check the calculation rather than accept it.

Effect on core layout and unit mix

A second stair is not simply another stair. It brings a protected lobby, firefighting provision where applicable, and the space to connect both stairs to the accommodation without compromising either escape route.

Two arrangements dominate. Two separate cores spread across the plan give better escape geography and cost more in circulation, because each core serves fewer units. A single enlarged core containing two stairs keeps circulation compact but needs careful separation so that a single incident cannot affect both routes.

The consequences follow quickly. Fewer units per floor for the same floor plate, or the same unit count on a larger floor plate. Corner units are often the first to go, which affects the proportion of dual aspect dwellings and therefore compliance with housing quality policy. Where a scheme relied on a particular unit mix to meet an affordable housing policy or a space standard, that mix has to be rechecked rather than assumed.

Net to gross ratio is the number to watch in the appraisal. A change of a few percentage points across a tall building is a material change in saleable area, and it affects viability, levy liability and the affordable offer together.

Implications for an existing permission

If the permission has not been implemented, the question is how to amend it. Three routes exist and they are not interchangeable.

A non-material amendment under section 96A of the Town and Country Planning Act 1990 suits changes with no material effect. Adding a stair core rarely qualifies, because it changes internal layout, usually changes the external envelope, and often changes the unit count.

A section 73 application to vary a condition, commonly the condition listing the approved drawings, is the usual route for a minor material amendment. It produces a new permission sitting alongside the original, and it cannot be used to change the description of development. So if the description specifies a number of dwellings and the stair reduces that number, section 73 may not be available.

A fresh full application is needed where the change goes beyond what section 73 can carry, or where the description has to change. That resets consultation, committee and in some cases the obligations in the section 106 agreement.

Decide the route early and discuss it with the case officer. An application submitted under the wrong route loses more time than the discussion would have cost.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

Evidence to submit

A submission dealing with a second staircase should carry five things, and the first two do most of the work.

A revised Fire Statement explaining the escape strategy with two stairs, prepared by someone with the necessary fire engineering competence. Revised plans at every level showing both stairs, their lobbies and the accommodation they serve. A schedule of accommodation comparing the consented and proposed unit mix. An updated net internal area and net to gross statement. And, where massing changes, an assessment of the external effects, which may include daylight, sunlight and townscape depending on the scale of the change.

Where the building's fire strategy as a whole is being revisited, a Fire Safety Strategy to BS 9991 or BS 9999 is a fire engineering document requiring a competent professional. A licensed or competent professional is legally required for this work, and an AI-assisted draft is a starting point for that professional rather than a substitute for their judgement.

An illustrative example

A consented 19 storey residential building of 61 metres has a single core with one stair and two lifts, serving nine units per floor. Redesign to two stairs within an enlarged core reduces the floor plate to seven units per floor, with the loss of two corner units and a fall in net to gross of around three percentage points. Because the description of development specifies the number of dwellings, a fresh application is submitted rather than a section 73, with a revised affordable housing offer supported by an updated appraisal. This example is illustrative rather than a real scheme.

What to do before the next design stage

The sequence that avoids abortive work is short. Establish the measured height against the Approved Document definition and record the margin. Confirm the commencement and transitional position for your programme. Test the two-stair core on the actual floor plate rather than on a diagram. Then recheck the unit mix, the housing quality compliance and the appraisal in that order, because each depends on the one before it.

One honest uncertainty: this part of the regulatory framework has changed repeatedly since 2020, and further change is possible. Treat any date quoted in an older consultant report as unreliable and check the current position directly.

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