Firefighting shafts in an office to flats conversion
A storey above 18 metres means a firefighting shaft, and office cores rarely have one. The lobby, stair and lift requirements, their effect on unit counts, and the consent route for the works needed.
If the office you are appraising has a storey more than 18 metres above fire service vehicle access level, it needs a firefighting shaft, and the existing core very rarely provides one. That single point kills more office to residential conversions than daylight, noise or space standards combined. The firefighting shaft conversion problem is geometric: a compliant shaft needs a protected stair, a lobby at every level and a firefighting lift, and an office core built for two passenger lifts and a single escape stair has nowhere to put them.
Work this out at appraisal stage. Once you own the building, the answer does not improve.
Why existing office cores rarely work
Offices are designed around escape, not around firefighting access. A 1980s or 1990s office of eight or nine storeys typically has one or two escape stairs reached directly off the open floorplate, lifts opening into the same space, and no lobbies. That arrangement is efficient for lettable area and entirely reasonable under the fire strategy for an office with simultaneous evacuation.
Convert the same building to flats and the fire strategy changes underneath you. Residential buildings are designed for stay put, with each flat as a fire compartment, a protected common corridor and a protected stair. Firefighters need a bridgehead two storeys below the fire, reached by a lift that is not the passenger lift you happen to have.
The three things an office core usually lacks are lobbies between the accommodation and the stair, a lift meeting firefighting lift requirements with its own protected lobby and power supply, and a rising main within the shaft.
What a firefighting shaft has to contain
Approved Document B sets the thresholds and the components. A firefighting shaft is required where a building has a storey at a height of more than 18 metres above fire service vehicle access level, or more than 10 metres below it. The shaft comprises a firefighting stair, a firefighting lobby at each level served, and, above 18 metres, a firefighting lift.
The dimensional consequences are the ones that hit your area schedule:
- A firefighting lobby with a minimum floor area of 5 m² at every storey served by the shaft.
- A firefighting stair of at least 1,100 mm clear width.
- A firefighting lift with its own lobby, secondary power supply and firefighter switch.
- A fire main within the shaft: a dry main serves buildings above 18 metres, and a wet main is needed where the highest floor is more than 50 metres above access level.
- Enclosure to the shaft in construction of substantial fire resistance, with self-closing fire doors at each junction.
Add the second staircase question. New residential buildings with a top storey above 18 metres are now expected to have two staircases under the amended Approved Document B, and the transitional arrangements that allowed single-stair schemes to continue run out at the end of September 2026. Check the current version of Approved Document B and its transitional wording before you rely on a single stair, because the answer on a specific site turns on when the building work started rather than when the planning application went in.
Lobby depths and the effect on net area
Here is the arithmetic developers tend to skip. A 5 m² lobby at each level is the minimum, and a practical lobby with door swings, a corridor connection and a lift landing usually lands between 8 m² and 12 m². Over nine storeys that is upwards of 70 m² removed from saleable area, taken from the part of the floorplate closest to the core, where the flat layouts are already tightest.
Then check the travel distances. Approved Document B limits single-direction escape in the common areas of a block of flats to 7.5 metres from a flat entrance door, which forces either a central core with short corridors or a second stair at the far end of the plan. Deep office plans with a single central core frequently cannot satisfy both the lobby requirement and the travel distance without losing a whole line of units.
An illustrative example, and it is illustrative rather than drawn from a real scheme: a 1,000 m² gross office floorplate with a central core of 90 m² might yield ten flats on paper. Insert a firefighting lobby, widen the stair to 1,100 mm and take a lift out of passenger service for firefighting duty, and the same floor yields eight, with two units losing their best aspect. The scheme still works, but the appraisal built on ten units does not.
Options where the core cannot be enlarged
Four routes exist, in rough order of how often they succeed.
- Extend the core outwards. A new shaft added to the flank or rear of the building, connected at every level, is the cleanest engineering answer. It is also external building operations, which matters for the consent route.
- Sacrifice a lift. Converting one of two passenger lifts to a firefighting lift costs less than new structure, though it depends on the existing lift shaft dimensions, machine room position and whether a protected lobby can be formed around the landing.
- Reduce the height. Removing the top storey or two from the residential scheme so that no storey sits above 18 metres takes the building out of the firefighting shaft requirement altogether. Painful on yield, and sometimes the only viable answer.
- Re-plan around a new internal shaft. Taking a strip of floorplate at each level for a new stair and lobby works where the frame allows large openings and the structural engineer can transfer loads. Expect a structural survey and a scheme of localised strengthening.
Each of these interacts with the consent route, which is the part that surprises people.
What prior approval under Class MA cannot change
Class MA of Part 3 of Schedule 2 to the General Permitted Development Order 2015 permits a change of use from commercial, business and service use to dwellings, subject to prior approval. It permits a change of use only. It does not authorise external building operations, so a new external stair core, a new lift overrun, new external doors at ground level or a reconfigured entrance all need separate planning permission.
The prior approval matters themselves include the impact on intended occupiers in relation to fire safety, which gives the authority a direct route into the question. Add the Gateway One regime: Article 9A of the Development Management Procedure Order 2015 requires a fire statement with applications for relevant buildings, meaning residential buildings of 18 metres or more in height or seven or more storeys, and the Health and Safety Executive is a statutory consultee at that stage.
A Fire Statement at Gateway One has to be prepared by someone with fire engineering competence. This is not a document a developer or architect should write alone. The same applies to the detailed Fire Safety Strategy that follows under BS 9991: both are cases where a competent professional is legally required, and an AI-assisted draft is a starting point for that professional rather than a substitute for them. Escape and fabric pull against each other in older buildings, a tension we look at in our piece on fire safety in listed buildings.
Testing feasibility before you buy
Run this sequence before exchange, and expect it to cost less than a week of contractor preliminaries.
- Measure the height of the top occupied storey above the access level the fire service would use, not above the pavement at the front door.
- Get a fire engineer to mark up the existing core drawings against Approved Document B, identifying where a lobby could be formed and whether either lift shaft can serve as a firefighting lift.
- Ask the structural engineer whether new openings and a new shaft are achievable in the existing frame, and at what cost.
- Rework the area schedule with the lobbies in place and the travel distances checked, then re-run the appraisal on the reduced unit count.
- Decide the consent route: prior approval for the change of use with a separate application for external works, or a single full planning application covering everything.
- Book pre-application advice on the external works, because that is where the design conversation happens.
The buildings that convert well are the ones with a generous core, a shallow plan and a storey height that keeps the top floor under 18 metres. And the ones that stall are usually nine-storey towers bought on a floor area calculation with no fire engineer in the room.
Browse all 66 PlanScope report types and start a draft, then send the fire documents for review through the professional directory before anything goes to the authority.
This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. For specialist technical reports, qualified professional input may be required. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
At what height is a firefighting shaft required?
Approved Document B requires a firefighting shaft where a building has a storey more than 18 metres above fire service vehicle access level, or more than 10 metres below it. Above 18 metres the shaft must include a firefighting lift as well as a firefighting stair and a lobby at each level served.
How large does a firefighting lobby have to be?
The minimum floor area for a firefighting lobby in Approved Document B is 5 m². In practice a workable lobby with door swings, a lift landing and a corridor connection tends to occupy between 8 m² and 12 m², which is area taken from the most constrained part of a converted office floorplate.
Does Class MA prior approval cover the new core?
No. Class MA permits a change of use only, so external building operations such as a new stair core, lift overrun or reconfigured entrance need separate planning permission. Fire safety for intended occupiers is one of the prior approval matters, so the authority can examine the internal position too.
Do converted buildings above 18 metres now need two staircases?
New residential buildings with a top storey above 18 metres are expected to provide two staircases under the amended Approved Document B, with transitional arrangements that run out at the end of September 2026. Check the current Approved Document B and the transitional wording, because the position depends on when building work starts.
Who has to write the fire statement at Gateway One?
Someone with fire engineering competence. Article 9A of the Development Management Procedure Order 2015 requires a fire statement with applications for relevant buildings of 18 metres or more or seven or more storeys, and the Health and Safety Executive is a statutory consultee. A draft can set the structure, but the author must be competent.
Is it ever better to reduce the building height?
Sometimes. Taking the residential scheme below 18 metres removes the firefighting shaft requirement and the Gateway One fire statement duty in one move. On a deep-plan building where no compliant shaft can be formed, losing the top storey can recover more units across the remaining floors than it costs.
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.