Firefighting shafts in a single core apartment plan

A firefighting shaft is a protected enclosure containing a firefighting stair, a firefighting lobby at every storey it serves and, where the building is tall enough, a firefighting lift, together with the fire main. On a single core apartment building that package is the single largest fixed element on the floor plate, and once the lobby is added at every level the core grows by an area that comes straight out of saleable accommodation. Test it on the plan before the massing is agreed, because the alternative is discovering at technical design that the scheme no longer works.

Approved Document B requires a firefighting shaft where a building has a floor more than 18 metres above fire service vehicle access level, or more than 10 metres below it. Those two thresholds decide the whole core strategy.

What a firefighting shaft comprises

Four components, and the lobby is the one designers underestimate.

The firefighting stair, protected through the full height, with its own enclosure. The firefighting lobby at each storey served, entered from the accommodation and giving access to the stair and the lift. The firefighting lift where the height thresholds are crossed, within the same protected enclosure and served by the same lobby. And the fire main, with an outlet in the lobby at each level.

The lobby has to be ventilated, which means either a smoke shaft with its own plan area and vent at roof level, or a mechanical smoke control system with a shaft, plant and a power supply. On an internal core with no external wall, that shaft is an additional plan item and it cannot be moved around freely once the structural grid is set.

Which version applies to a particular building is a fire engineering judgement, and a Fire Safety Strategy to the relevant British Standard is an engineering document rather than a drafting exercise, so it needs a fire engineer. An AI-assisted draft is a starting point for that engineer, never a substitute.

Core area and the planning consequence

The arithmetic is unforgiving on a small floor plate. Stair, lift shaft, lobby, smoke shaft and riser cupboards, each with structural walls around them, consume a fixed area at every level, and that area does not shrink as the floor plate does.

So the efficiency of a single core tower depends almost entirely on the ratio between core area and gross floor area. A larger floor plate carries the core comfortably. A slender tower on a tight urban site may reach a point where the net to gross ratio makes the scheme unviable, and the response is usually a wider footprint, which is a planning question about massing, daylight to neighbours and townscape.

Get that tested before the pre-application meeting. A massing that has been agreed in principle with an officer and then has to grow by two metres in each direction to accommodate a compliant core is a difficult conversation, and it is avoidable.

The second staircase question now sits alongside this. Government has confirmed a requirement for a second staircase in new residential buildings above 18 metres, with transitional arrangements, so check the current text of Approved Document B and the transition dates before assuming a single core is available at all. On a scheme above that height the planning strategy should assume two cores unless the transition clearly applies.

Lobby arrangement and unit access

Dwellings are entered from the common area, and the relationship between that common area and the firefighting lobby determines whether the plan works.

Two arrangements are used. Dwellings entered from a common corridor that opens into the firefighting lobby, which keeps the lobby small but lengthens travel distances. Or dwellings entered directly from the lobby itself, which shortens travel but makes the lobby larger and puts front doors onto a firefighting route.

The second arrangement is tighter on area and raises more questions from a fire consultee, because the lobby's function during an incident conflicts with it being the access to occupied dwellings. Where it is used, the fire strategy has to explain the smoke control and the door arrangements in detail.

Either way, check the travel distance from the furthest point of every dwelling to the lobby door, and check the hose laying distance. Approved Document B expects every part of a storey to be within 60 metres of a fire main outlet, measured along a route suitable for laying hose rather than in a straight line. On a long floor plate served by one shaft, that figure is what limits the plan length, and it frequently requires a second shaft on a building that would otherwise have one.

Effect on units per core

Work the number out from the constraints rather than from a target.

Start with the hose distance from the shaft, which fixes the maximum reach of the floor plate in each direction. Then apply the travel distance limits from dwelling entrance doors. Then check that the resulting corridor or lobby arrangement is deliverable with the smoke control you intend. What is left is the number of dwellings per storey that the core can serve.

On a typical single core residential building the answer lands at a modest number per floor, and schemes that assume more usually do so by ignoring either the hose distance or the lobby ventilation. Running this calculation at concept stage is the least costly piece of fire engineering advice available, and it should happen before the unit mix is fixed in the planning submission.

Service risers within the shaft

The protected shaft containing the firefighting stair is not a service riser. Services other than those permitted, and gas pipes in particular, do not belong in it, and a design that routes risers through the stair enclosure to save space will be corrected.

So allow separate riser cupboards accessed from the common area, for water, electrical distribution, data, heating and ventilation, plus the sprinkler or suppression riser where provided. Each needs access for maintenance and fire-resisting enclosure, and each takes plan area next to the core.

The commonest coordination failure on these plans is a riser cupboard that opens into the firefighting lobby. Keep maintenance access out of the lobby, and show the cupboard doors on the submitted floor plans so the arrangement is legible.

Coordination with the fire statement

On a relevant building the fire statement submitted at Planning Gateway One has to describe the core strategy, so the information has to exist at application stage rather than being deferred.

A Fire Statement is a document where competence is legally expected under the Building Safety Act 2022 framework and Article 9A of the Development Management Procedure Order 2015. It must be prepared by someone with the necessary fire engineering competence, and the dwelling count and height that bring a scheme into the gateway are easy to misjudge, as how the gateway threshold applies to educational accommodation shows.

Make the drawings and the statement say the same thing. A floor plan showing a single stair with no lobby, submitted alongside a statement describing a compliant firefighting shaft, tells the consultee that the two documents were produced separately.

Testing the plan early

Five checks, each of which takes an hour and each of which has stopped a scheme at technical design:

• draw the 60 metre hose route from the proposed fire main outlet on the worst floor plate, following the actual route rather than a radius
• measure travel distances from the furthest habitable room in the furthest dwelling to the lobby door
• draw the smoke shaft or the mechanical shaft at its real size, including its route to roof level and the plant space
• add the riser cupboards at a realistic size with doors onto the common area
• calculate the net to gross ratio with all of the above, on the smallest floor plate in the scheme

Then put the result in front of the fire engineer before the design and access statement is written. Explaining in the Design and Access Statement why the core is the size it is, and why the footprint follows from it, is far more persuasive than presenting a massing and defending it later.

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, and this article is general guidance rather than advice on a specific building. Enter your postcode on PlanScope to see which reports your scheme is likely to need.