In 2025, 131,523 flats changed hands in England and Wales — 16.1% of the 814,445 residential sales Land Registry recorded that year, and 97.7% of those flats sold leasehold. A meaningful share of that flat stock exists because, at some point, somebody took a single house and turned it into two or more self-contained homes. Doing that legally takes two separate approvals that most buyers have never heard of — and getting one doesn't mean the other happened too.

Splitting a house into flats is a change of use, not an extension

Most people's mental model of "planning permission" comes from extensions: build out, build up, maybe need permission, maybe don't under permitted development rights. Subdividing a house into separate flats is a different category of decision entirely. It's a material change of use — turning one dwelling into several — and that almost always requires full planning permission from the local planning authority, regardless of whether any building work happens at all.

This matters because permitted development rights, which let homeowners make many common alterations without applying for permission, are built around extending or altering a single dwelling. They don't generally extend to creating additional, independent dwellings out of one. Planning permission in England and Wales is the general starting point for checking what a specific proposal needs, but the practical rule of thumb is consistent across local authorities: more self-contained homes than you started with means an application, not an assumption.

The building regulations sign-off nobody asks about

Planning permission answers one question — is the local authority prepared to allow this change of use. It says nothing about whether the finished flats are physically safe to live in, which is a completely separate legal track: building regulations approval.

Converting a house into flats typically triggers at least two building regulations requirements that don't come up in a normal extension:

  • Fire separation between units. Once strangers (or at least separate households) share a building, the fire safety standard changes — compartmentation between flats, protected escape routes and, often, upgraded doors become necessary under Approved Document B.
  • Sound insulation between flats. New floors and walls separating one flat from another generally need to meet specific sound resistance standards under Approved Document E, which simply don't apply to a single house.

Building control sign-off is obtained independently of planning permission, through either the local authority's building control team or an approved inspector. A conversion can clear planning committee and still never get a completion certificate if the fire and sound insulation work wasn't signed off — or, just as often, the other way round: building regulations work gets done and approved years before anyone checks whether planning permission was ever applied for.

Two systems, two timelines — and council tax makes it three

Planning permission, building regulations and council tax banding are three independent processes run by three different parts of the same council, on three different timelines, triggered by three different things. What happens to your council tax when a house is split into flats covers the third one in detail: the Valuation Office Agency can band each new flat for council tax the moment it becomes a separate "hereditament" with its own occupier, independently of whether planning enforcement has caught up with an unauthorised conversion, or whether building control ever signed it off.

In practice this means a council can quite happily be charging four separate council tax bills on a four-flat conversion that technically never had planning permission. The three systems don't check each other automatically — a local authority department discovering an unauthorised conversion typically does so through a neighbour complaint, a subsequent planning application, or a council tax enquiry that happens to flag an unusual pattern, not through some shared register that cross-references all three processes against each other.

What the numbers say about how common this is

Flat conversions aren't a fringe case in the housing stock — they're a meaningful slice of it. Leasehold tenure, in particular, is almost universal among flats specifically because splitting a freehold house into separately-owned units typically requires creating leasehold interests for each one. Searching a postcode on Homecost shows this pattern clearly in areas with a lot of Victorian and Edwardian terraced housing, like Highbury in north London, where large original houses have frequently been split into two or three leasehold flats, each now trading and taxed as a separate home.

What to check before you buy a converted flat

None of this is a reason to avoid converted flats — most are entirely lawful, built, inspected and banded correctly, with the paperwork to prove it. But because the three systems don't automatically reconcile with each other, it's worth specifically asking for:

  • Evidence of planning permission for the conversion (or written confirmation it wasn't required for a given property), rather than assuming it was granted because the flat already has its own council tax band.
  • A building regulations completion certificate covering the fire separation and sound insulation work specifically, not just a general certificate for unrelated works.
  • How the current council tax band was set and when, since a band can be assigned well before — or after — the other two processes are resolved.

If any of this is unclear, a conveyancer can raise specific enquiries with the seller and, where needed, the local planning authority before exchange, or advise on indemnity insurance where older conversions lack full paperwork. A related situation — what happens to a single council tax band when a house is extended rather than split — is covered in does an extension change your council tax band?

This is general information about how the planning and building control systems work, not planning or legal advice for a specific property. A conveyancer, planning consultant, or the local planning authority can confirm the position on an individual conversion. For more buyer guidance on costs that follow you after completion, see Homecost's buyer guides.