Smoke control shafts and plant space on planning drawings

Smoke control is a planning problem disguised as a building regulations problem. The shaft itself takes floor area out of every storey. The equipment that drives it takes a room at roof level or at the base, with a louvre on an elevation and a plant enclosure above the parapet. None of that appears in a concept design unless someone has put it there deliberately, and the version drawn after planning permission is granted rarely fits inside the approved envelope.

On any residential building with a common corridor, decide the smoke control principle before the massing is fixed. On a building at or above 18 metres, decide it before the core is fixed, because the firefighting shaft and the smoke shaft together set the core width.

Why smoke control affects the planning layout

Three separate consequences, each of which shows on a planning drawing.

The shaft is a vertical hole through the building, typically adjacent to the stair core, continuous from the lowest storey it serves to the roof. It consumes the same footprint on every floor, it cannot be offset without serious penalty, and it reduces the net area available for dwellings. On a small floorplate with two or three flats per core, losing one to two square metres per storey to a shaft changes the unit mix.

The equipment needs a room. A natural shaft needs a ventilator at the head with an actuator and a control panel. A mechanical system needs a fan room, usually at roof level, with duty and standby fans, acoustic treatment, a control panel and access for maintenance.

And the discharge needs to reach outside air. That means either a vent at roof level, which adds height and visible structure, or a louvre in an elevation at the right level with a clear path to open air, which adds a visible element on a facade that may face a street or a conservation area.

Natural against mechanical systems and their space demands

The two approaches trade floor area against roof-level bulk, and the trade runs in opposite directions.

Verify the dimensional requirements against the current Approved Document B and the fire engineer's design, because the figures above describe the standard prescriptive arrangement and an engineered solution may depart from them for good reasons.

The practical point for a planning submission is that the choice is not free. A developer who chooses mechanical ventilation to recover floor area on every storey has bought a fan room and a plant enclosure at roof level. On a site where height is already contentious, that can be the wrong trade.

Shaft position within the core

Smoke shafts belong next to the thing they ventilate, which is the common corridor, and the corridor leads to the stair. That puts the shaft in or adjacent to the core, competing with the lift, the stair, the risers, the refuse chute and, above 18 metres, the firefighting lift and its lobby.

Three positioning rules that save redesign:

• keep the shaft within the core zone rather than between flats, because a shaft between dwellings needs fire separation on both long sides and creates awkward residual spaces
• align it vertically through every storey, including any podium or commercial level, and check it does not land on a transfer structure or a basement wall
• give it an access route at the head and a maintenance route at the base, both of which need to appear on the plans

Where the building is above 18 metres, the firefighting shaft is the dominant constraint and the smoke shaft has to be coordinated with it. A core sized without both at concept stage typically grows by a metre in one direction during technical design, which on a tight site either loses a unit or pushes the building footprint beyond the approved line.

Fan rooms, louvres and the elevation

This is where planning permissions get breached.

A roof-level fan room is a structure. It has a footprint, a height, a louvred face for air movement, a door, and a requirement for the fans to be accessible for replacement. It sits alongside the lift overrun, the water tank, any air source heat pumps, the photovoltaic array, the maintenance access and the balustrade to the roof terrace if there is one. The sum of those is a roof plant zone, and the planning drawings need to show it as a designed element rather than as a dashed outline labelled "plant".

Four things to resolve before submission.

The height of the plant enclosure above the main parapet, because that is what determines whether the building reads as the height shown on the street elevation or taller. Where height is constrained by policy or by a daylight and sunlight assessment, test the plant zone in the model.

The treatment of the enclosure, which in a conservation area or next to a listed building will be assessed on its own terms. A screened enclosure in a material that relates to the building generally survives. Galvanised louvres on a stick frame generally do not.

Louvre positions on the elevations, including any intake at low level, with free area stated. Louvres added later are a material change to the approved elevations.

Visibility from viewpoints. If a townscape and visual impact assessment or verified views have been prepared, the plant zone needs to be in the model that generated them, or the views do not represent what will be built.

Coordination with the escape strategy

The smoke control system exists to keep the escape route usable, so it only makes sense alongside the escape strategy, and the planning layout has to be consistent with both.

The questions that change the plan: how many stairs the building needs, whether the corridor arrangement requires a single shaft or more than one, whether any dwelling has a travel distance that needs an additional protected route, and whether the building requires a firefighting shaft. Each of those alters the core, and the core alters the floorplate.

On taller buildings the national policy position on second staircases has moved in recent years and continues to be worked through in the Approved Documents and in guidance. Check the current position and the applicable transitional arrangements for your scheme before fixing a single-stair layout, because a scheme designed with one stair that later needs two loses far more than shaft space.

What to show on the submitted drawings

The submission that avoids a later amendment includes the following.

1. Floor plans showing the smoke shaft on every storey, dimensioned, with its fire-rated enclosure indicated.
2. A roof plan showing the full plant zone to scale, including the fan room, its access and its louvres, with heights annotated.
3. Elevations and sections showing the plant enclosure and any louvres, drawn rather than hatched, with the enclosure height dimensioned from a stated datum.
4. A section through the core showing the shaft continuity from base to head.
5. A short note in the design statement explaining the smoke control principle and the reason for the chosen approach.
6. Where the building is a relevant building for the purpose, a Fire Statement addressing the matters the regulator is consulted on at the planning stage.

A Fire Statement is a document where fire-engineering competence is legally required, so an AI-assisted draft is a starting point for that professional rather than a substitute for their judgement. The same applies to a Fire Safety Strategy prepared to BS 9991 or BS 9999, which is a fire-engineering document in its own right and sits downstream of the planning submission.

Queries the consultee raises

The questions that come back are predictable, so answer them in the submission.

Whether the plant zone shown is the whole plant zone, or whether more will be added. Where the smoke shaft discharges, and whether the discharge is clear of openable windows, terraces and the intake for any ventilation system. Whether fire service access and the hydrant position work for the building as laid out, including appliance hardstanding and the route from the appliance to the firefighting shaft entrance. Whether the system has a life safety electrical supply and where the standby provision sits. And, on a building near 18 metres, the measured height, because the measurement convention decides which regime applies.

The last one deserves a specific check. A scheme drawn at 17.8 metres that acquires a parapet upstand, a thicker roof build-up or a raised plant plinth during technical design has changed regime, and with it the consultation requirements and the building control route. Fix the height measurement early and record how it was taken.

Where the shaft and plant coordination interacts with the structure or the ground, the related documents are set out in the report index, and the structural and basement interfaces are covered in a structural appraisal of a building proposed for retention and existing foundations and basement depth.

This is general guidance, not legal advice; check the position with your local planning authority, building control 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.