Firefighting shafts and the floorplate they consume
The shaft is a package, not a stair: firefighting stair, lobby at every level, lift and smoke shaft. On a small floorplate the area it removes can decide viability.
A firefighting shaft floorplate problem is an area problem disguised as a fire safety one. The shaft is not a stair. It is a protected enclosure containing a firefighting stair, a firefighting lobby at every level it serves, a firefighting lift with its machine space, and the rising main outlets, and it takes that area out of every storey in the building. On a small floorplate, adding one can remove enough lettable or saleable area to change the scheme's viability.
Which is why the fire engineer belongs in the room while the core is being laid out, not after planning permission.
What a firefighting shaft contains
Approved Document B and the British Standards treat the shaft as a package rather than as a stair with extras. The components:
- a firefighting stair, protected and of a stated minimum width
- a firefighting lobby between the stair and the accommodation at every storey served
- a firefighting lift, where the height threshold requires one, with its own protected enclosure
- the lift machine space, which has to be accessible and may need its own protected route
- rising main outlets within the lobby, so a crew can connect hose in a protected space
- ventilation of the stair and lobby, by a shaft or a mechanical system
The lobby is the component people forget. It is not the lift lobby and it is not a landing. It is a protected space sized so a crew can stage, connect to the main and work, and the standards set both a minimum and a maximum floor area for it. Too small and it fails. Too large and it becomes unprotectable.
Mechanical smoke ventilation to the shaft adds a riser and a plant space at roof level, and that riser also runs through every storey.
The thresholds that require one
Height is the obvious trigger. A firefighting shaft is required where a storey sits above a stated height over fire service vehicle access level, and the lift becomes mandatory at the same sort of threshold. Depth triggers it too, for basements below a stated level.
Less well known is the area trigger. Buildings other than blocks of flats with a large storey at a relatively modest height can require a shaft on floor area grounds alone, and deep basements of substantial area can require one regardless of the height of the building above. A two-storey distribution shed with a large mezzanine can therefore need a shaft where a four-storey office does not.
Read the current Approved Document for the figures. The thresholds have moved, the residential and non-residential volumes differ, and the measurement datum is fire service vehicle access level rather than any floor level in the building. On a sloping site, the datum is where the appliance will stand, which can be a storey below the main entrance.
The area it takes on every storey
The arithmetic is worth doing early, on the back of an envelope, before anyone draws a unit layout.
An illustrative example, with the dimensions assumed for illustration rather than taken from a real scheme. A shaft comprising a stair enclosure of around 15 square metres, a firefighting lobby of around 7 square metres, a lift shaft of around 5 square metres and a smoke shaft of around 2 square metres occupies roughly 29 square metres per storey once the enclosing wall thicknesses are counted. Over twelve storeys that is around 350 square metres of gross area, and all of it is non-lettable.
Put that against the floorplate and the picture changes by scale. On a 1,200 square metre floorplate it is under 3 per cent. On a 450 square metre floorplate it is over 6 per cent, before the general circulation, the second stair, the service risers and the refuse chute. Add those and a small floorplate tower can lose a third of its gross area to the core.
Two consequences for the appraisal. The net to gross ratio has to be tested at the planning stage, not assumed from a previous scheme with a different floorplate. And the loss is proportionally worse on the smallest floors, which on a tapering or set-back building are the upper floors where values are highest.
The second stair requirement alongside it
The requirement for a second staircase in new residential buildings above a stated height now sits alongside the firefighting shaft requirement, and the two interact.
The practical questions are whether the firefighting stair can count as one of the two, whether both stairs need firefighting lobbies, and how the two stairs are separated. The answers come from the current Approved Document and the relevant British Standard, and the position has changed within the last few years, including transitional arrangements for schemes already in the system. Confirm where your scheme sits rather than relying on a precedent from a 2022 submission.
What is clear is the area effect. A second stair of around 15 square metres per storey, with its own lobby where required, roughly doubles the vertical circulation take. On a scheme designed before the change, retrofitting it has meant losing a unit per floor or extending the footprint, and neither is a minor amendment.
So on any residential scheme approaching the threshold, test the two-stair arrangement at the same time as the single-stair one and submit the drawings that work under the current requirement.
Small floorplates and where the arithmetic fails
Below a certain floorplate, a code-compliant core leaves too little accommodation to be worth building. The figure is scheme-specific, and three factors move it.
Floorplate shape. A square plan puts the core in the middle efficiently. A narrow plan on a tight urban site forces the core to one end, which lengthens travel distances and may require a third protected route.
Unit mix. A floor of four two-bedroom flats tolerates a large core better than a floor of two larger units, because the common circulation per unit falls as unit count rises. Up to the point where travel distances from the furthest door force another measure.
Site constraints. A core cannot always go where the arithmetic wants it. A party wall, a sewer easement or a basement obstruction can push it to a position that costs more area.
The honest answer on marginal sites is that a scheme that only works on a non-compliant core does not work. Better to establish that in week two with a test fit than in month eight with a refused Gateway One submission.
What can and cannot be shared
Sharing is where the area is recovered, and the rules are narrower than developers hope.
| Element | Shareable | Note |
|---|---|---|
| Firefighting lobby and lift lobby | Generally not as a single space | The firefighting lobby is a protected staging space with its own requirements |
| Firefighting lift and passenger lift | Sometimes, if the lift meets the firefighting specification | Specification, power supply and protection all apply |
| Rising main between two stairs | Sometimes | Outlet positions must still achieve hose coverage on every floor |
| Smoke shaft serving stair and corridor | Depends on the strategy | A tested or modelled system may allow it |
| Firefighting stair as an escape stair | Usually yes | Subject to the second stair requirement and lobby provision |
The one that most often releases area is using a passenger lift that meets the firefighting lift specification, so the building has one lift group rather than a separate firefighting lift sitting idle. It carries a cost in specification and in protected power supply, and it has to be agreed with the fire engineer at concept stage because it affects the shaft dimensions.
Mechanical smoke ventilation can also recover area compared with a natural shaft, at the cost of a system that needs maintenance, standby power and a commissioning regime. On a tall residential building, a modelled system is often the arrangement that makes a tight core work. Material choices interact with all of this, and our note on mass timber schemes and the questions planners now ask covers how structure changes the fire case.
Testing the core before you submit
Five steps, in this order, before the planning drawings are frozen:
- Establish the fire service vehicle access level and the resulting height measurement, in writing
- Identify every trigger that applies, height, depth and area, and record the Approved Document edition
- Produce a test-fit core at one typical floor, with the lobby drawn at the required area and the wall thicknesses shown
- Calculate the net to gross for that floor and for the smallest floor in the building
- Have the fire engineer confirm the arrangement in a short written note, which becomes the basis of the Fire Statement
The written note is worth insisting on. Planning Gateway One brings the Building Safety Regulator in as a statutory consultee on relevant residential buildings, and the submission has to address fire service access, water supplies and the fire strategy. An application whose core does not work will be identified there, after the design fees have been spent.
For basement levels the shaft interacts with escape and with flood risk, which our note on basement light wells, escape and flood risk together deals with. Where the building falls below the Gateway One threshold, the same material belongs in a Fire Safety Strategy prepared by someone with fire engineering competence.
Browse the report types to see which fire and structural documents a tall building application usually carries.
This is general guidance, not legal advice. A Fire Statement and a fire safety strategy must 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. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
What does a firefighting shaft include?
A protected enclosure containing a firefighting stair, a firefighting lobby at every storey served, a firefighting lift with its machine space where the height requires one, rising main outlets within the lobby, and ventilation of the stair and lobby by shaft or mechanical system. The lobby is a protected staging space, not a lift lobby.
Is a firefighting shaft only needed in tall buildings?
No. Height above fire service vehicle access level is one trigger and basement depth is another, but floor area triggers one too. Buildings other than blocks of flats with a large storey at a modest height can require a shaft on area grounds, as can deep basements of substantial area.
How much area does a shaft remove?
Enough to matter on a small floorplate. Counting the stair, lobby, lift and smoke shaft with their enclosing walls, the take per storey runs to tens of square metres, repeated on every floor. On a floorplate under 500 square metres the core as a whole can absorb close to a third of gross area.
Can the firefighting stair count as the second staircase?
Usually, subject to the current requirements on second staircases in new residential buildings and on lobby provision to each stair. The position has changed recently and carries transitional arrangements, so confirm it against the current Approved Document rather than relying on a submission from a few years ago.
What can be shared to recover area?
The arrangement that most often releases area is a passenger lift specified to the firefighting lift standard, so the building has one lift group instead of an idle firefighting lift. It carries costs in specification and protected power supply, and it has to be agreed with the fire engineer at concept stage because it sets the shaft dimensions.
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.