Category A or Category B? How BSR classifies work in a high-rise flat

By Alicja Salter, Building Surveyor and BSR compliance consultant (MSc Building Surveying) · Last reviewed 26 October 2026

If you are planning work to a flat in a higher-risk building (HRB) in England, the Building Safety Regulator (BSR) will put it in one of two categories: Category A or Category B. Both need BSR building control approval before work starts. The category decides how much information you must submit with the application, and GOV.UK warns that applying under the wrong one is likely to get the application rejected.

This guide explains what each category means, how typical flat projects fall, and what the government's 2026 proposals might change. It is general information, not legal advice or a building control decision.

The short answer

Category A is work that touches anything on a fixed list in the regulations: layout, fire safety, escape routes, the external wall, common parts and more. It needs a full set of supporting documents.
Category B is everything else. It needs a lighter application, although BSR can ask for more.
Neither category means no approval. Unless your work is exempt, done under a competent person scheme or an emergency repair, you need BSR approval before you start (GOV.UK).

Where the categories come from

The categories are set out in regulation 12(6) of the Building (Higher-Risk Buildings Procedures) (England) Regulations 2023. Work is Category A if it falls within any one or more of these descriptions:

Work that increases or decreases the building's external height or width.
Work that changes the number of storeys, including adding or removing a mezzanine or gallery floor.
Work that changes the number of flats or residential rooms.
Work that changes the number or width of staircases or other escape routes.
Work to the external wall, apart from certain components excluded by regulation 7(3) of the Building Regulations 2010.
Work that changes the internal layout.
Work that affects passive fire safety measures, such as fire-resisting walls, doors, floors and fire stopping.
Work that affects active fire safety measures, such as sprinklers, smoke detection, alarms or smoke ventilation.
Work that affects the common parts not covered above.

The regulations define Category B simply as "work which does not fall within category A".

Why the category matters: the paperwork

For Category A work, regulation 12(2)(c) requires the application to include all of these:

a competence declaration
a construction control plan
a change control plan
a mandatory occurrence reporting plan
a building regulations compliance statement
a fire and emergency file (often called a fire strategy)
a partial completion strategy, if part of the building will be occupied before the work is finished

For Category B work, you include whichever of those documents you consider appropriate. Where the fire safety requirements of the Building Regulations apply, you must also include fire safety compliance information. BSR can then require any of the Category A documents by written notice before it decides (legislation.gov.uk).

GOV.UK guidance puts the fire document like this: Category A work needs a fire and emergency file, while Category B work that might affect fire spread may only need a fire compliance statement.

For both categories, BSR has up to 8 weeks to decide an application for work to an existing HRB, unless you agree a longer period in writing. In practice decisions often take longer: see How long does BSR approval take?

How typical flat projects usually fall

This is a guide to the usual answer, not a ruling. A small detail, such as a new opening in a fire-resisting wall, can move a job into Category A.

Typical work in a flat	Usually	Why
Moving, removing or adding an internal wall or door	A	Changes the internal layout
Knocking two flats into one, or splitting a flat	A	Changes the number of flats or residential rooms
Replacing the flat entrance door	A	Affects passive fire safety
Replacing or rewiring smoke or heat detection	A	Affects active fire safety
New service penetrations through compartment walls or floors	A	Affects passive fire safety (fire stopping)
Replacing windows	Depends	External wall work is Category A, but some components are excluded; take advice
Like-for-like kitchen or bathroom refit, no layout change, no new penetrations	Often B	May touch none of the nine descriptions
Work that also touches a corridor, riser or other common part	A	Affects the common parts

Two points catch people out:

Non-structural does not mean Category B. A layout change is Category A whether or not the wall carries load. See Can I remove or move a wall in a high-rise flat?
Fire safety reaches further than you think. Walls, doors, ceilings and service routes inside a flat often form part of the building's fire separation. Any work that affects them is Category A.

How to decide which category applies

Confirm the building is higher-risk. For existing buildings that means at least 18 metres or 7 storeys, with at least two residential units. See How do I find out whether my building is higher-risk?
Describe the work in full. Include enabling works: strip-out, new service routes, ceiling removal and making good.
Test it against all nine descriptions. If any one applies, it is Category A.
Check fire safety information early. Ask the building's accountable person for the fire risk assessment and any fire strategy, so you know which walls, doors and floors are fire-resisting.
If in doubt, get professional advice. A building surveyor, fire engineer or building control professional can help you choose the category and prepare the documents. See Which professionals do I need for a BSR application?

Starting with the category that fits the work avoids a rejection and a resubmission.

What the 2026 reforms might change

The government accepts that the current line catches too much small work. Its March 2026 consultation on categorising HRB work says the 2023 rules have unintentionally pulled some relatively small or minor works into Category A. BSR had received 1,944 Category A applications for non-cladding work, roughly 46% of all non-cladding applications. Replacing a single fire door in a flat was the example given.

It proposed two changes:

Option 1: take most work inside individual flats out of Category A. Work would stay in Category A if it affects active fire safety measures, load-bearing walls, or the flat's external walls (excluding windows). Fire doors between a flat and the communal areas would be considered separately.
Option 2: take small-scale communal work out of Category A, if it can typically be done by three or fewer people within five working days and avoids the same safety-critical elements.

The government response, published on 9 July 2026, says it is minded to exclude most work inside residential units from Category A and is still working out the detail (Wedlake Bell; DAC Beachcroft). As of October 2026 the law has not changed. Any change needs new regulations, and BSR approval will still be required before work starts.

Until the rules change, apply the current nine descriptions. Do not plan on the proposed rules.

The bottom line

Category A or B is about how much information BSR needs, not whether you need approval at all. Test your project against the nine Category A descriptions before you prepare the application, and build the approval time into your programme. If you are not sure where your project sits, the BSR screener gives you a first view in minutes.

Sources

Not sure whether your project needs BSR approval?Answer a few questions about your building and your planned work and get a PDF report on whether Building Safety Regulator approval is likely to be needed.

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