Joining two flats in a high-rise block: what BSR approval involves

Buying the flat next door, or the one above, and joining the two is a common way to get more space in a London block. In a higher-risk building (HRB) in England, a building of at least 18 metres or 7 storeys with at least two residential units, it is also one of the clearest cases for Building Safety Regulator (BSR) approval. This guide explains why, what the BSR will want to see, and what to sort out before you commit to a design. It is general information, not legal advice or a building control decision.

The short answer

• You will need BSR approval before any work starts. Joining two flats is building work to an existing HRB, and it is not exempt work (GOV.UK).
• It is always Category A. GOV.UK gives "joining two flats or splitting one" as an example of changing the number of residential units, which is Category A work.
• It usually triggers several Category A descriptions at once: the number of flats, the internal layout and passive fire safety, and sometimes the common parts.
• The building's owner is involved too. You will need your freeholder's consent under both leases, and the building's accountable person will want to know.

Why it is always Category A

Work to an existing HRB is either Category A or Category B. Both need approval; the category decides how much information goes with the application. The categories are set out in regulation 12(6) of the Building (Higher-Risk Buildings Procedures) (England) Regulations 2023. Work that changes the number of flats or residential rooms is one of the nine Category A descriptions, so a merger lands there whatever else it involves. See Category A or Category B? for the full list.

In practice a merger rarely touches just one description. It typically also:

1. Changes the internal layout. A new opening, a new internal stair or rearranged rooms all count, whether or not any wall is load-bearing. See Can I remove or move a wall in a high-rise flat?
2. Affects passive fire safety. The wall or floor between two flats is normally fire-separating construction. Cutting an opening through it changes how fire and smoke are contained.
3. May affect the common parts. If one front door onto the corridor is removed or sealed, or services in a shared riser are altered, the work reaches beyond your own flat.

Changing the number of dwellings in a building is also a "material change of use" under regulation 5 of the Building Regulations 2010. Your designer needs to show that the combined flat, taken as a whole, meets the relevant requirements, not just that the new opening is built properly.

The fire safety questions

Fire safety is where most of the design effort goes, because the two flats were designed as separate fire compartments. Expect the BSR to look closely at:

• The opening between the flats. The wall or floor between them is currently fire-separating construction. Once the flats become one home, the design must show how the new opening, and what remains of that wall or floor, works with the rest of the building's fire strategy.
• Escape from the larger flat. A bigger flat can mean longer routes to the front door. Many flats rely on a protected entrance hall, and a duplex adds an internal stair. Your fire engineer or designer must show how people get out safely from every room.
• The front door you no longer need. Keeping both doors, sealing one or replacing one all affect the fire separation between the flat and the corridor. Flat entrance doors are part of that separation. See Do I need approval to replace my flat entrance door or windows?
• Alarms and detection. Two separate systems may need to become one system that covers the whole flat. Work that affects active fire safety measures is a separate Category A description.
• Fire stopping. Every pipe, cable and duct that passes through walls or floors that still separate you from neighbours or the corridor must be properly sealed.

Structure

The wall between side-by-side flats is often structural, particularly in concrete-framed and cross-wall blocks, and the floor between stacked flats almost always is. Cutting through either needs a structural engineer's design and, often, a survey of the existing structure first. GOV.UK says the BSR expects applicants to evidence how the work affects the existing building through surveys or inspections, rather than relying on assumptions (GOV.UK).

Services

Two flats come with two of everything: electricity supplies and meters, heating systems, water supplies, and sometimes ventilation routes. Decide early which you will keep and which you will remove. Disconnections and new routes often involve shared risers or other common parts, which brings in the freeholder and may add to the Category A work. Any electrical or heating work that would otherwise be done under a competent person scheme goes into the main application, because GOV.UK says exempt or scheme work that forms part of a wider project must be included in that project's application.

What goes in the application

For Category A work, the application must include, among other things (legislation.gov.uk; GOV.UK):

• drawings of the existing and proposed flats and a description of the work
• a building regulations compliance statement
• a fire and emergency file (often called a fire strategy)
• a construction control plan and a change control plan
• a mandatory occurrence reporting plan
• a competence declaration from the client

The application must also name a principal designer and a principal contractor. For a merger, the team usually includes an architect or building surveyor, a structural engineer and a fire engineer. See Which professionals do I need for a BSR application?

BSR has up to 8 weeks to decide an application for work to an existing HRB, unless a longer period is agreed. In practice, decisions often take longer. See How long does BSR approval take?

The other permissions

BSR approval is only one of the consents involved:

• Freeholder consent. You hold two leases, and both will normally restrict alterations. Joining the flats may also need the leases varied or merged, which is a legal question to raise early with your solicitor. See Lease consent, freeholder consent and BSR approval.
• The accountable person. Each HRB has an accountable person responsible for building safety in the common parts, usually the freeholder or management company. A merger changes the building they are responsible for, so talk to them before you finalise the design.
• Planning. Some London boroughs have policies resisting the loss of homes, including where flats are combined. Check with your local planning authority before you commit.

Before you start

1. Confirm the building is higher-risk. See How do I find out whether my building is higher-risk?
2. Get early feasibility advice on the structure and fire strategy before you finalise the design.
3. Talk to the freeholder and accountable person about consent, the leases and any work to common parts.
4. Treat the project as Category A and allow time for the full application.
5. Do not start any work, including strip-out, until approval is granted. Starting without it is a criminal offence. See What happens if I start work without BSR approval?

If you are weighing up a purchase next door, the BSR screener gives you a first view of whether your plans are likely to need BSR approval and what that means for your programme.

Sources

• Building control approval for higher-risk buildings, GOV.UK (Building Safety Regulator), updated 1 October 2026
• Preparing information for a building control approval application, GOV.UK
• The Building (Higher-Risk Buildings Procedures) (England) Regulations 2023, regulation 12, legislation.gov.uk
• The Building Regulations 2010, regulation 5, legislation.gov.uk