Fire safety in conversions of commercial buildings to flats

Class MA prior approval does not assess fire safety. The matters the General Permitted Development Order lists for a commercial to residential conversion cover transport, contamination, flooding, noise from commercial premises, natural light and, in some cases, the loss of the ground floor use. Fire is absent. That does not mean fire safety is optional, it means the control sits elsewhere: in the Building Regulations, and at planning stage only where the building triggers the fire statement requirement.

For an investor buying an office or a retail block to convert, that split is the main source of unpleasant surprises. The planning route can be cleared in 56 days and the building control route can then prove the scheme undeliverable at the density the appraisal assumed.

What the prior approval route does and does not assess

The prescribed matters are a closed list, so the authority cannot refuse prior approval because the escape strategy is inadequate or the external wall construction is unknown. It can refuse because habitable rooms would not receive adequate natural light, and the layouts that cause fire problems in a deep office floorplate often cause daylight problems too, so the two issues arrive together in practice.

The fire statement requirement is separate and comes from Article 9A of the Town and Country Planning (Development Management Procedure) (England) Order 2015. It applies to applications involving a relevant building, broadly one containing two or more dwellings or educational accommodation that is 18 metres or more in height or has seven or more storeys. Whether a prior approval application for such a building requires a statement is a question to put to the authority in writing before submission rather than assume either way, because the answer affects validation.

So the planning permission and the deliverability of the building are decided by two different regimes, on two different timetables, by two different sets of people.

Existing cores and means of escape

The commonest deal-breaker is the stair. An office building is designed for a population that is awake, familiar with the building and evacuates simultaneously. A residential building is designed for occupants who are asleep, unfamiliar with their neighbours' flats and expected to stay put while the fire service attends.

Three consequences for a conversion.

1. A single stair serving a deep floor plate may be acceptable for an office and unacceptable for flats, because travel distance limits from a flat entrance door to the stair are shorter and the lobby arrangement is more demanding.
2. The stair enclosure's fire resistance and the lobby protection are often absent in an older office, where open atria, unprotected stair landings and glazed internal screens were the point of the design.
3. Approved Document B now requires a second staircase in new residential buildings above 18 metres, subject to transitional arrangements. A conversion is not a new building, but the direction of travel affects what a building control body will accept and what a warranty provider will insure, so check the current edition and take advice before relying on a single stair at height.

The practical test on a site visit is simple. Walk from the furthest point of each floor to the stair, count the doors, and look at what the stair is enclosed in. Where any part of the scheme puts habitable rooms at or below ground level, the escape geometry tightens further, as our note on safe egress from habitable basement rooms sets out.

Compartmentation in a building not designed for it

Flats need compartmentation between each other and between each flat and the common parts. Offices do not, so the structure you are buying almost always lacks it.

The problem areas are consistent. Raised access floors and suspended ceilings, which create continuous voids across what will become party lines. Service risers without fire stopping at each floor. Ductwork crossing new compartment walls. Curtain walling with no cavity barriers at floor level. And a structural frame where fire protection to beams and columns was specified for an office occupancy.

Each of those is solvable and each costs money and ceiling height. The ceiling height point is the one that moves appraisals: adding a fire-rated floor build-up and a protected ceiling in a building with a 2.6 metre slab-to-slab dimension can leave a finished height that is uncomfortable or non-compliant with local policy.

External wall construction on an older frame

This is the diligence item most often skipped and the one most likely to be fatal to a deal. Buildings from the 1980s onwards may have rainscreen or curtain wall systems with insulation and cavity arrangements that cannot be identified from drawings, and drawings for refurbished buildings are frequently wrong.

Commission an intrusive survey before exchange, not after. Open up at several locations, record the build-up, and identify insulation products, cavity barriers and the fixing arrangement. Where the building is above 18 metres, the restrictions on combustible materials in external walls apply to the works being carried out, and remediation is a seven-figure item on a large frontage.

Where the survey cannot be done before exchange, price the risk explicitly or make the purchase conditional. An assumption that the facade is compliant because the building has a current occupation certificate is not diligence.

Where a fire statement is triggered

Height and storey count are the triggers, and they are measured in a defined way, so a building at the margin needs the measurement checked rather than estimated. Mezzanines, plant enclosures and sloping ground all affect the answer.

Where the statement is required, it must be prepared by someone with the necessary fire engineering competence. A Fire Statement is legally required in these cases rather than merely advisable, and the same is true of a Fire Safety Strategy prepared to the relevant British Standard. PlanScope can draft the planning narrative and document structure as a starting point for that professional, and it is not a substitute for one. Buildings sitting just under the threshold raise their own evidential questions, covered in our note on fire evidence for a building just below the threshold.

Coordinating planning and building control

Run them in parallel, with the fire engineer engaged before the prior approval application rather than after. The reason is that the fire-driven layout changes, such as a second escape stair or a protected corridor, change the unit count and the natural light position, and natural light is a prescribed matter the authority will assess.

A sequence that works on most conversions:

1. Measured survey and intrusive facade survey.
2. Fire engineer's initial appraisal of escape, compartmentation and external wall, with a view on deliverable unit count.
3. Test layouts against both the fire constraints and the daylight requirement.
4. Submit the prior approval application on the layout that satisfies both.
5. Building control submission and, where applicable, the regulatory gateway process for a higher-risk building.

Skipping step two is what produces a granted prior approval for a layout the building cannot deliver, and prior approval is tied to the submitted details, so a materially different layout needs a fresh application.

Evidence to gather before you buy

Six items, and all six are obtainable during a normal diligence period: the original and any refurbishment drawings, a measured survey with slab-to-slab dimensions, an intrusive facade survey, an asbestos survey, a fire engineer's appraisal, and confirmation from the authority on whether a fire statement will be required for a prior approval application on this building.

The asbestos position in particular affects the programme for opening up compartment lines, and it interacts with the demolition and strip-out sequence, as our note on demolition asbestos surveys and the planning submission sets out.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. A Lawful Development Certificate is the only formal way to confirm that a use or works are lawful. 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.