Firefighting shafts and what they take from the floor plate
Once a floor sits more than 18 metres above fire service access, the core gains a lobby, a lift and a smoke shaft with minimum dimensions that do not compress.
A firefighting shaft is the single largest fixed cost to the floor plate in a mid-rise building, and it arrives at a height most residential schemes cross without noticing. Approved Document B expects a firefighting shaft where a building has a floor more than 18 metres above fire service vehicle access level, or a basement storey more than 10 metres below it. Once the trigger is met, the firefighting shaft core stops being a lift and stair and becomes a package of protected spaces with dimensional minimums attached.
Designers who set the core size from a stacking diagram before the fire engineer is appointed usually lose between one and two saleable units per floor putting it right.
What a firefighting shaft contains
The shaft is a protected enclosure containing a firefighting stair, a firefighting lift where the height requires one, and a firefighting lobby between the stair and the accommodation on each storey. The rising main sits inside it, with outlets in the lobby.
Each element has a minimum. The firefighting lobby needs a free area of at least 5 square metres, which is larger than the circulation lobby many schemes draw. The firefighting lift car has a minimum size and load, ordinarily 1,100 by 1,400 millimetres with a rated load of 630 kilograms and a door clear width of 800 millimetres. The firefighting stair has its own minimum width.
Then the parts that get forgotten. Smoke control to the lobby, which is either a natural shaft of stated cross-sectional area or a mechanical system with its own plant and riser. A protected power supply to the lift and to any mechanical smoke control. And access from the lobby to the rising main outlet with room to work.
When one is required
The primary trigger is height: a floor more than 18 metres above the level of fire service vehicle access. The basement trigger is a storey more than 10 metres below that level, which catches deep basements under buildings that are otherwise low.
Dry rising mains serve buildings up to a height threshold, above which a wet rising main with its own tank and pump becomes necessary. That tank and pump room is a second claim on low level floor area, and it is separate from any sprinkler tank the scheme also needs.
Two related thresholds run alongside and are often confused with this one. Automatic sprinkler protection in new blocks of flats is triggered at 11 metres to the top storey, below the firefighting shaft threshold. Higher-risk building status under the Building Safety Act 2022 applies at 18 metres or seven storeys, which broadly coincides with the shaft trigger but is a different regime with its own gateways.
Dimensional consequences for the core
Work the core up from the lobby rather than down from a target net-to-gross figure.
| Element | Typical minimum | What it competes with |
|---|---|---|
| Firefighting lobby | 5 m² free area per storey | Lift lobby, flat entrance doors |
| Firefighting lift | 1,100 x 1,400 mm car, 630 kg | Passenger lift sizing and shaft width |
| Firefighting stair | Protected, minimum width per guidance | Escape stair width for occupancy |
| Smoke shaft | Natural shaft of stated area, or mechanical plant | Riser space, roof plant area |
| Rising main | Within the shaft, outlets in the lobby | Lobby working space |
The lobby is the binding constraint on small plots. Five square metres of free area, with doors swinging into it and a rising main outlet on the wall, sets a minimum core width that does not reduce however efficient the rest of the plan is.
On a plan of around 400 square metres per floor, a compliant firefighting shaft with its lobby and smoke shaft takes a share of the plate that is material rather than marginal. That is the number to put in front of the client at feasibility, not after planning.
Interaction with evacuation lifts and second stairs
A firefighting lift and an evacuation lift are different provisions with different purposes. The firefighting lift is for firefighters under fire service control. An evacuation lift is for occupants who cannot use stairs, managed under an evacuation strategy.
A single lift can sometimes be designed to serve both roles, and it needs deliberate design rather than an assumption. Where both are provided separately, the core grows again.
The second staircase requirement for taller new residential buildings has been through consultation and amendment, with transitional arrangements attached, and the threshold has moved during that process. Check the current Approved Document B text and the applicable transitional provisions rather than working from a remembered figure, because a scheme designed to the wrong version is a redesign rather than a revision.
Effect on saleable area
The commercial point is simple and worth stating plainly. Every square metre in the shaft is a square metre that is never sold or let, on every floor, for the life of the building.
Three consequences follow for the development appraisal. Net-to-gross ratios from a scheme below 18 metres do not transfer to one above it. A building that crosses the threshold by half a metre carries the full cost of the shaft, so the section is worth testing against the threshold before the massing is fixed. And where the scheme is close to the line, the decision between 17.8 metres and 18.4 metres is a commercial decision with a fire engineering input, not the other way round.
Test the alternative honestly. Dropping a storey to stay below the trigger loses units. Crossing it loses area on every floor and adds a tank, a pump and a smoke control system. Which wins depends on the plate size, and on a small plate the shaft is proportionally more painful.
Options on constrained plots
Where the plate is tight, the moves that work are about combining functions rather than shrinking minimums.
- Use the firefighting stair as one of the means of escape stairs, which is permitted and normal, rather than providing a separate stair.
- Put the smoke shaft directly against the lobby so it does not need a separate riser run through the plate.
- Locate the core against a party wall or a blank elevation, so the lobby does not consume perimeter with daylight value.
- Share the lift core geometry between the firefighting lift and a passenger lift, sizing both to the firefighting standard.
- Reconsider the basement. A basement more than 10 metres deep triggers the shaft on its own, and a shallower basement with a different servicing strategy may avoid it entirely.
Point five is the one most often available on a scheme that is otherwise below 18 metres. Where a deep basement is unavoidable, a Basement Impact Assessment will be needed for the planning application in any event, and the scope of that work is set out in basement impact assessment scope and stages.
Evidence at application stage
Planning does not approve the fire strategy, and the planning drawings still have to be capable of accommodating it.
Where the building is a relevant building, meaning a residential building of 18 metres or more or seven storeys, a Fire Statement is required with the application under Article 9A of the Development Management Procedure Order 2015 and the Building Safety Regulator is consulted at Planning Gateway One. That statement must be prepared by someone with the necessary fire engineering competence under the Building Safety Act 2022 framework, and an AI-assisted draft is a starting point for that professional rather than a substitute.
Show the core at the right size on the submitted plans. A permission granted on drawings with a 3 square metre lobby produces either a non-compliant building or an application to amend, and amendments that change the external envelope are not non-material. The wider strategy sits in the fire safety strategy, which should exist in outline before the planning drawings are frozen.
This is general guidance, not legal advice, so check the current Approved Document B and take fire engineering advice 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.
Frequently asked questions
When is a firefighting shaft required?
Approved Document B expects one where a building has a floor more than 18 metres above the level of fire service vehicle access, or a basement storey more than 10 metres below it. The basement trigger catches deep basements under buildings that are otherwise low rise, and it is the one most often missed at feasibility.
How big does a firefighting lobby have to be?
At least 5 square metres of free area on each storey served. That is larger than the lift lobby many schemes draw, and with doors swinging into it and a rising main outlet on the wall it sets a minimum core width that does not reduce however efficient the rest of the plan becomes.
Is a firefighting lift the same as an evacuation lift?
No. A firefighting lift is for firefighters under fire service control during an incident. An evacuation lift is for occupants who cannot use stairs, operating under an evacuation strategy. A single lift can sometimes serve both roles, but only through deliberate design rather than assumption, and providing both separately grows the core.
Does the firefighting shaft threshold match the sprinkler threshold?
No. Automatic sprinkler protection in new blocks of flats is triggered where the top storey is more than 11 metres above ground level, well below the firefighting shaft trigger. Higher-risk building status under the Building Safety Act 2022 applies at 18 metres or seven storeys, which is a separate regime again.
Can the firefighting stair also be an escape stair?
Yes, and it normally is. Providing a separate stair for firefighting alone is rarely necessary and wastes plate. The stair has to meet the firefighting stair requirements, including protection and the lobby arrangement, and its width also has to satisfy the escape calculation for the occupancy it serves.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.