Second staircases in tall residential blocks: planning effects

A second staircase in a residential building above 18 metres is a Building Regulations requirement, not a planning one. It still lands on the planning file, because the extra core changes the floorplate, the unit mix and often the massing, and because a building of that height needs a fire statement at Planning Gateway One before the application can be validated.

If you are holding a consent designed around a single stair and the scheme is not yet built, the question is not whether the stair goes in. It is whether the change can be made under section 73 or needs a fresh application.

Where the second staircase requirement comes from

The 18 metre threshold runs through the whole building safety regime. It defines a higher-risk building under the Building Safety Act 2022, alongside the seven storey alternative, and it is the height at which the Building Safety Regulator becomes the building control authority.

Second staircases sit in guidance to the Building Regulations rather than in the Act. Approved Document B was amended following the 2023 consultation on sprinklers and stairs, to expect a second staircase in new residential buildings above 18 metres, with transitional arrangements for schemes already in the pipeline. Those transitional provisions are the part that decides whether your scheme is caught, and they have been amended since first publication. Check the current Approved Document B and its transitional wording on gov.uk before you fix a programme, rather than relying on a summary written when the change was announced.

A second staircase is a fire engineering matter throughout. A Fire Safety Strategy to BS 9991 has to be prepared by someone with the necessary fire engineering competence, and PlanScope's draft is a starting point for that professional rather than a substitute for them.

How a second core changes the floorplate

The area lost is more than the stair enclosure. A compliant second escape stair brings with it a protected lobby, a smoke control route, fire fighting shaft provision where required, and the structural transfer of the core down through the building.

An illustrative example, using round numbers to show the method rather than to state a norm. Take a stair enclosure of 18 m² and its associated lobby and riser space at 12 m², so 30 m² per storey. On a 20 storey block that is 600 m² of gross internal area redirected from lettable space to circulation. At an average flat size of 65 m², the arithmetic points to roughly nine flats. Whether the real figure is higher or lower depends entirely on the geometry, and on a slab that was already inefficient it can be worse.

The second-order effects usually hurt more than the area. Two cores on a rectangular plate push flats to the ends and cut the number of dual aspect corner units. A single-stair tower built around a central core may not be able to take a second stair at all without moving from a point block to a slab block, which is a different building and a different planning judgement.

Unit mix and efficiency consequences

Expect three things to move together.

• Total unit count falls, which affects viability, affordable housing quantum and any section 106 heads of terms already agreed
• The mix shifts towards smaller units, because the remaining plate suits one and two bedroom flats better than three bedroom family units
• Dual aspect percentages fall, which matters where policy sets a target

That combination is what triggers a policy problem. A scheme consented with 25 per cent three bedroom units can end up at 15 per cent after the stair goes in, and the housing mix policy in the adopted local plan is what the officer will measure it against. Deal with it head on in the covering statement, with the arithmetic set out, rather than letting a case officer discover it.

Where the reduction is material, revisit the affordable housing position early. A reduction in units alters both the obligation and the viability appraisal that supported it.

Amending a consented scheme, and choosing the route

There are three routes, and they are not interchangeable.

Adding a second stair to a consented block is almost never a section 96A. Whether it fits within section 73 depends on how the permission is worded. If the description of development is broad, for example a number of dwellings and a maximum height, and the drawings are controlled by condition, a section 73 to substitute revised drawings is usually workable. If the description recites a specific building configuration, section 73 will not reach it.

The Levelling-up and Regeneration Act 2023 introduced a further route at section 73B for applications to vary a permission where the effect is not substantially different. The date it came into force and the detail of how authorities are applying it have moved, so check the current position before choosing it over section 73.

Whichever route you take, the fire statement has to be revisited. A Fire Statement is required for relevant buildings under Article 9A of the Development Management Procedure Order 2015, and the Health and Safety Executive is a statutory consultee at Planning Gateway One. A statement prepared for a single-stair scheme does not describe the building you are now proposing.

Fire statement content for a two-stair building

The statement is a planning document about how fire safety has shaped the design, not a technical fire strategy. For a two-stair building it should be explicit about six things.

1. The height and storey count, and why the building is a relevant building for Article 9A purposes.
2. The number and location of stairs, and how they are separated so that a single incident cannot compromise both.
3. Fire service access to the building, the position of the fire fighting shaft and the appliance standing position.
4. Evacuation strategy, and whether it is stay put, simultaneous or phased, with the design consequences of that choice.
5. Provision for the evacuation of disabled residents, including any evacuation lift and its lobby.
6. External wall construction and the approach to materials, at the level of detail the design has reached.

The Building Safety Regulator's comments come back through the local planning authority, and they are directed at whether fire safety has been properly considered in the design, rather than at approving the strategy. Approval comes later, at Gateway Two.

Programme and cost implications

Three timing points are worth building into the appraisal.

The consenting route is the long pole. A fresh application on a major scheme runs to a 13 week determination period before any extension of time, and Gateway One consultation sits inside that. A section 73 on a major scheme carries the same statutory period.

Construction cost rises for reasons beyond the stair. A second core usually means additional lift provision, more smoke shafts, additional fire fighting main capacity and structural transfer at podium level.

And the sequencing of Gateway Two matters. Where the design changes materially after planning permission, the building control approval application to the Building Safety Regulator has to reflect the final design, and a change made late is a change made twice.

Structural implications of a new core through an existing frame need surveying rather than assuming, which is where a Structural Survey earns its place on a conversion or a partially built scheme.

PlanScope drafts fire and structural planning documents structured to UK planning policy, with a "to be confirmed" flag wherever design information is missing. Fire Statements and Fire Safety Strategies legally require input from someone with fire engineering competence, and the draft is a starting point for that professional. This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional 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.