Section 257 HMO Licensing in Hounslow: Converted Buildings and Blocks of Flats

Most landlords in Hounslow think of HMO licensing as something that applies to shared houses — three tenants sharing a kitchen, that sort of arrangement. But the Housing Act 2004 defines HMOs in two distinct ways, and the second one catches a type of property many landlords don't expect: converted blocks of flats.

Section 254 vs Section 257: Two Different HMO Definitions

Section 254 is the standard HMO test. A property qualifies as an HMO if it's occupied by three or more persons forming two or more separate households, and those occupants share one or more basic amenities — a kitchen, bathroom, or toilet. This is the definition that catches shared houses, flatshares, and bedsit-style accommodation. In Hounslow, properties with three or four occupants from multiple households need an additional HMO licence, while those with five or more need a mandatory licence.

Section 257 works differently. It applies to buildings that have been converted into self-contained flats, where the conversion didn't comply with the Building Regulations in force at the time (or the 1991 Regulations, whichever is more recent) and still doesn't meet that standard. If less than two-thirds of the flats in the building are owner-occupied (a long lease of 21+ years or freehold counts as owner-occupation), the whole building is classified as an HMO — even though each flat is self-contained.

The distinction matters because a Section 257 HMO can trigger licensing obligations even when no individual flat contains sharers.

How Section 257 Applies Under Hounslow's Additional Licensing Scheme

Hounslow's borough-wide Additional HMO Licensing Scheme (4) 2025 does cover Section 257 converted buildings, but with an important local restriction: a Section 257 block only requires an additional licence if the entire building and all the self-contained flats within it are under the same single ownership or effective single control.

In practice, this means:

• Caught: A landlord owns a Victorian house converted into four flats (conversion pre-dates or doesn't meet the 1991 Building Regs), lets all four, and owns the freehold. The building is under single ownership, so it needs an additional HMO licence.
• Not caught: The same converted building, but two of the four flats have been sold on long leases. More than a third of the flats are owner-occupied, so it falls outside the Section 257 definition entirely.
• Also not caught: Three different landlords each own one flat in a converted building. There's no single ownership or control, so Hounslow's additional licensing scheme doesn't apply — even if the building meets the Section 257 structural test.

What About Mandatory Licensing?

Section 257 converted buildings are explicitly excluded from mandatory HMO licensing under the 2018 Prescribed Description Order. The mandatory scheme only applies to HMOs as defined by Section 254 — the shared-amenity test. So even a large converted block with many tenants won't need a mandatory licence under the five-or-more-persons rule. It might, however, need an additional licence if Hounslow's single-ownership condition is met.

Checking Whether Your Converted Building Needs a Licence

Start by answering these questions:

1. Was the building converted into self-contained flats? If it's purpose-built flats (designed and constructed as flats from the start), Section 257 doesn't apply.
2. Did the conversion comply with the Building Regulations at the time (or the 1991 standard)? If it did and still does, Section 257 doesn't apply.
3. Are two-thirds or more of the flats owner-occupied (long lease 21+ years or freehold)? If yes, Section 257 doesn't apply.
4. Is the entire building under single ownership or effective single control? If not, Hounslow's additional licensing scheme doesn't cover it even if it's technically a Section 257 HMO.

If the answer to questions 1–3 brings the building within Section 257 and the single-ownership condition in question 4 is met, you need an additional HMO licence from Hounslow Council.

Where there's genuine uncertainty — mixed ownership structures, partly-compliant conversions, or complex head-lease arrangements — request written confirmation from Hounslow's Housing Enforcement Team at [email protected] before committing to an application.

Our Hounslow landlord licensing guide covers the full decision tree for all three licensing categories, including the Section 257 single-ownership test and the documents you'll need for an application.

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