The most expensive mistake in this area is assuming that because a building already exists and already has a fire risk assessment, changing what happens inside it is a matter for the fire risk assessment.
It is not. Change of use and many fit-outs are building work and building work engages the Building Regulations. This explains what triggers that and what it means.
The critical distinction
A fire risk assessment evaluates the risk to people in a building as it is used, under the Regulatory Reform (Fire Safety) Order 2005. It is retrospective and operational.
Building Regulations apply to work. Where work is carried out, it must comply with the applicable requirements and compliance is demonstrated to a building control body, usually through a fire strategy.
The two are not alternatives. If you carry out work that engages Part B and never involve building control, you have unauthorised building work and no fire risk assessment cures that.
What counts as a material change of use
Regulation 5 of the Building Regulations 2010 defines it. There is a material change of use where there is a change in the purposes for which or the circumstances in which, a building is used so that after the change:
- (a) the building is used as a dwelling, where previously it was not
- (b) the building contains a flat, where previously it did not
- (c) the building is used as an hotel or boarding house, where previously it was not
- (d) the building is used as an institution, where previously it was not
- (e) the building is used as a public building, where previously it was not
- (f) the building is not a building described in classes 1 to 6 in Schedule 2 (exempt buildings), where previously it was
- (g) the building, containing at least one dwelling, contains a greater or lesser number of dwellings than previously
- (h) the building contains a room for residential purposes, where previously it did not
- (i) the building, containing at least one room for residential purposes, contains a greater or lesser number of such rooms
- (j) the building is used as a shop, where previously it was not
- (k) the building becomes a building described in regulation 7(4)(a), being a relevant building for external wall material purposes
"Institution" is defined as an establishment, whether described as a hospital, home, school or other similar establishment, used as living accommodation for or for the treatment, care or maintenance of, persons suffering from disabilities due to illness, old age or other physical or mental incapacity or persons under the age of 18, where such persons sleep on the premises.
Worked examples
| Project | Material change of use? |
|---|---|
| Office block to flats | Yes, regulation 5(a) and (b) |
| House to two flats | Yes, regulation 5(b) and (g) |
| Six-flat block converted to eight flats | Yes, regulation 5(g) |
| House to HMO with rooms for residential purposes | Yes, regulation 5(h) |
| Warehouse to gym | Depends. Not automatically listed but may be a public building under 5(e). Check the specific circumstances |
| Warehouse to retail | Yes, regulation 5(j) |
| Pub to restaurant | Generally no change of use under regulation 5 but the works may still be building work |
| Office to office, new tenant, partitions installed | Not a change of use but likely building work as material alteration |
| Shop to café | Not listed but the works may be building work |
| Any of the above where the building becomes a relevant building under regulation 7(4) | Yes, regulation 5(k) |
Note that the Building Regulations definition is not the same as the Town and Country Planning use classes. A project can require no planning permission, for example under permitted development rights for office to residential conversion and still be a material change of use for Building Regulations purposes.
That combination, permitted development plus material change of use, is where a great deal of poor-quality conversion has occurred.
What a material change of use requires
Regulation 6(1)(a) provides that where there is a material change of use of the whole of a building, such work as is necessary must be carried out to ensure the building complies with, in all cases:
- B1 means of warning and escape
- B2 internal fire spread, linings
- B3 internal fire spread, structure
- B4(2) external fire spread, roofs
- B5 access and facilities for the fire service
alongside a list of other requirements covering condensation, ventilation, water supply, sanitary provision, drainage, waste storage, combustion appliances, conservation of fuel and power and electrical safety.
Regulation 6(1)(b) additionally applies A1 to A3, structure, where the change is to an hotel or boarding house, an institution, a public building or from an exempt building.
Regulation 6(1)(c) applies B4(1), external fire spread over walls, in the case of a building exceeding eleven metres in height. That threshold was reduced from 18 metres with effect from 1 December 2022.
Regulation 6(3) provides that where the change is described in regulation 5(k), such work as is necessary must be carried out to ensure that any external wall or specified attachment contains only materials of class A2-s1, d0 or class A1, other than the items exempted by regulation 7(3).
Where the change of use affects part only of a building, regulation 6(2) applies those requirements to that part, except that where regulation 6(1)(c) applies, the whole building must comply with B4(1).
That last point catches people. Converting the top two floors of a twelve-metre building into flats can engage external wall requirements for the entire building, not just the converted part.
Why this matters more than people expect
A material change of use requires the building to comply with B1 to B5 as they apply. It is not a matter of leaving the building as it is and writing a fire risk assessment about it.
That means demonstrating:
- Means of escape appropriate to the new occupancy, which for a conversion from awake-and-familiar office occupancy to asleep-and-unfamiliar residential occupancy is a fundamentally different problem
- Compartmentation appropriate to the new use, which in a residential conversion means each flat as a compartment
- Detection and alarm appropriate to the new use
- Fire service access and facilities
The change in occupancy characteristic is the crux. Under BS 9999, office occupancy is characteristic A, occupants awake and familiar with the building. Residential occupancy is characteristic C, occupants likely to be asleep. Everything downstream changes: travel distances, protection of escape routes, detection, compartmentation.
An office floor plate that was perfectly compliant as an office is frequently nowhere near compliant as flats and no amount of fire risk assessment makes it so.
Fit-outs: when is it building work?
Regulation 3 defines building work to include, among other things, the erection or extension of a building, the material alteration of a building and work required by regulation 6 in relation to material change of use.
A material alteration is, broadly, work which would result in a building or controlled service or fitting no longer complying with a relevant requirement where it previously did or being more unsatisfactory in relation to such a requirement than before.
In practice, for a fit-out:
Likely to be building work:
- Partitioning a floor plate in a way that affects escape routes or travel distances
- Forming or blocking openings in fire-resisting construction
- Removing or altering compartment walls or floors
- Installing a mezzanine
- Altering or extending a fire alarm or emergency lighting system in a way that affects compliance
- Creating inner rooms
- Changing the layout of an escape route
- Installing new stairs or removing existing ones
- Anything penetrating fire-resisting construction for services
Less likely to be building work:
- Redecoration
- Loose furniture
- Non-structural, non-fire-resisting demountable partitions that do not affect escape
The dividing line is not always obvious and "we are only putting up a few partitions" is not a reliable assessment of it. Where a fit-out changes travel distances, exit capacity or compartmentation, it needs to be checked.
The Approved Document B point about linings
B2 applies to internal linings, requiring them to resist the spread of flame and, if ignited, have a rate of heat release or fire growth reasonable in the circumstances. Fit-outs frequently introduce new wall and ceiling linings, feature panelling, acoustic treatments and decorative finishes without anyone checking the reaction to fire classification.
This is one of the more common findings in post-fit-out fire risk assessments and one of the more expensive to remedy once installed.
What you should do
Before design
- Establish whether the project is a material change of use under regulation 5
- Establish whether the works are building work under regulation 3
- Obtain the existing fire strategy. If there is none, consider a retrospective strategy before designing because you cannot safely alter a building whose design basis you do not know
- Establish the new occupancy characteristic and what it implies
During design
- Produce a fire strategy for the building as it will be, not a note appended to the existing one
- Identify every compartment line the works affect and how it will be maintained or replaced
- Check external wall requirements where the building exceeds eleven metres
- Check linings against B2
Before starting
- Make the building control approval application. Where Part B imposes a requirement, four copies of the full plans are required rather than two because the building control body must consult the fire and rescue authority
- For higher-risk buildings, this is Gateway 2 with the Building Safety Regulator
At completion
- Discharge regulation 38: hand the fire safety information to the responsible person, obtain the acknowledgement notice and notify the relevant authority
- Update or commission the fire risk assessment for the building as now used
The enforcement risk
Two things worth knowing.
Building control has powers in relation to unauthorised building work and under the Building Safety Act 2022 amendments the time limit for a section 36 notice requiring removal or alteration of non-compliant work was extended from 12 months to 10 years. Work carried out quietly in 2026 remains actionable in 2035.
Separately, the fire and rescue authority enforces the Fire Safety Order. Where a conversion has produced a building with inadequate means of escape, a prohibition notice under Article 31 can prohibit or restrict use immediately and Article 31 expressly extends to domestic premises other than a house occupied as a single private dwelling. For a residential conversion, that means residents out.
Frequently asked questions
Is converting an office to flats a material change of use? Yes, under regulation 5(a) and (b). It engages B1 to B5 and, where the building exceeds eleven metres, B4(1) for the whole building.
Do I need building regulations approval if I have permitted development rights? Yes. Planning and building control are separate. Permitted development removes the need for planning permission, it does not remove Building Regulations compliance.
Is a fire risk assessment enough for a change of use? No. A fire risk assessment discharges the Article 9 duty under the Fire Safety Order. It does not demonstrate compliance with the Building Regulations and building control will not accept one in place of a fire strategy.
Does converting a house to an HMO trigger the Building Regulations? It may, under regulation 5(h) where rooms for residential purposes are created and the works themselves may be building work in any event. It also engages the Fire Safety Order for the common parts and the Housing Act 2004 regime. Take advice on all three.
We are only installing partitions. Do we really need a fire strategy? If the partitions affect travel distances, exit capacity, compartmentation or escape route protection, the works need to be assessed against Part B. Whether that produces a full fire strategy or a shorter design note depends on the scale.
What if the previous owner did the conversion without approval? That is a due diligence finding and potentially an inherited liability. Section 36 notices can now be served up to 10 years after the work. Establish what was done, when and whether it was approved, before you buy.
