Second staircases in tall residential buildings

The second staircase requirement for new residential buildings above 18 metres sits in the Building Regulations guidance, not in planning policy, and that is exactly why it causes trouble. A scheme can hold a valid planning permission for a single-stair tower and still be undeliverable, because the building control route will not accept it. The fix is almost always a revised planning application, since a second core changes the plan, the elevations and often the unit count.

The amended guidance in Approved Document B takes effect on 30 September 2026, with transitional arrangements for work already in the building control system. If you are holding a consented single-stair scheme, the question to answer this month is whether you sit inside those arrangements or outside them.

Where the requirement comes from

It came out of the response to the Grenfell Tower inquiry and the wider review of fire safety in tall residential buildings. Government consulted in 2022 and 2023 on sprinklers and stair provision in residential buildings, initially proposing a 30 metre threshold, then announced in 2023 that the threshold would be 18 metres.

The amendment to Approved Document B was published in 2024 with an implementation date of 30 September 2026, giving the industry a long lead-in. In London, the Greater London Authority had already set its own interim expectation ahead of the national change, so schemes in the capital have been designing to a second stair for longer. Check the current Mayoral guidance for the London position, which is not identical to the national one.

Approved Document B is guidance rather than law. Compliance is with the functional requirements of the Building Regulations, and an alternative approach supported by fire engineering is possible in principle. In practice, with the Building Safety Regulator acting as building control authority for higher-risk buildings, a single-stair proposal above the threshold is a difficult conversation to have.

The threshold and how height is measured

The trigger is 18 metres, and the measurement matters as much as the number.

Building height under Approved Document B is measured from ground level to the floor surface of the topmost storey, excluding any storey that consists exclusively of plant or machinery. That definition catches out schemes designed to the roof parapet or to the top of the plant enclosure, which can put a building several metres either side of the line.

The 18 metre figure aligns with the definition of a higher-risk building under the Building Safety Act 2022, which is 18 metres or at least seven storeys containing at least two residential units. A building can hit seven storeys below 18 metres, which is why both limbs need checking.

There is no planning threshold here at all. The planning system engages through the fire statement required at Planning Gateway One under Article 9A of the Development Management Procedure Order 2015 for relevant buildings, which is a separate obligation covered in our note on Gateway One fire statements.

Transitional arrangements

Transitional provisions turn on the building control process rather than the planning permission. Broadly, buildings where a building control approval application or initial notice was submitted before the implementation date, and where work is sufficiently progressed within the period allowed, can proceed under the earlier guidance.

Two practical points follow.

A planning permission granted before the implementation date gives you nothing on its own. The transitional test is about building control, so a consented scheme that has not entered the building control process is outside the arrangements.

The words "sufficiently progressed" carry a specific meaning in the transitional wording, and it is not the same as having a contractor on site. Read the published transitional provisions in the current Approved Document at the date you rely on them, and take advice on whether your scheme meets the test, because getting this wrong means redesigning a building already under construction.

What it does to consented schemes

Adding a second staircase to a designed building is not a detail change. The core grows, the corridor arrangement changes, and the net to gross ratio falls on every floor.

The last row is where the planning case gets difficult. A developer recovering lost area by adding a storey is proposing a taller building than the one the council assessed, which reopens daylight and sunlight to neighbours, townscape and sometimes protected views.

Amending the permission

A second staircase is very unlikely to be a non-material amendment under section 96A. It changes the internal layout, the elevations and usually the schedule of accommodation.

The realistic routes are a section 73 application to vary the approved drawings condition, where the change can be characterised as a variation rather than a different development altogether, or a fresh full application where the change is substantial or where units are being lost or gained.

Prepare for the section 106 conversation early. A reduction in units changes the affordable housing contribution and often the other obligations, and councils will want the deed varied rather than simply carried across. On a scheme where viability has moved since the original consent, expect that conversation to take longer than the determination itself.

Where the building crosses 18 metres as a result of the redesign, or crosses it for the first time, the fire statement obligation and the higher-risk building regime apply from that point, which is a further reason to settle the height early.

Coordinating with the fire strategy

The stair is one element of an evacuation strategy, not the strategy itself. The design has to be coordinated as a whole:

1. Fix the evacuation strategy first, whether stay put, phased or simultaneous, because it drives stair capacity, alarm provision and the need for an evacuation lift.
2. Confirm the firefighting shaft arrangement, the rising main and the fire service access, since these often dictate core position more than the stairs do.
3. Establish compartmentation and the smoke control approach, which affects lobby size and therefore core area.
4. Check whether an evacuation lift is required or expected, since it consumes core area that is easy to overlook in an early layout.
5. Bring the fire engineer into the planning stage rather than after consent, because core position is a planning matter once it changes the elevation.

A Fire Statement submitted at Gateway One is required at Planning Gateway One for relevant buildings under the Building Safety Act 2022 framework and Article 9A of the Development Management Procedure Order 2015, and it must be prepared by someone with the necessary fire-engineering competence. An AI-assisted draft is a starting point for that professional rather than a replacement for them.

One honest uncertainty: how the Building Safety Regulator will treat fire-engineered alternatives to a second stair on buildings marginally above the threshold is not yet settled by any body of published decisions. Designing to the guidance is the low-risk course, and a scheme relying on an alternative approach should have that conversation before planning rather than after.

Follow changes as they are announced on the planning law updates timeline, and browse the 66 PlanScope report types to see which documents a tall residential scheme needs.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.