C4, large HMOs and where sui generis begins

The C4 HMO use class boundary sits at six residents. Class C4 of the Use Classes Order covers the use of a dwellinghouse by not more than six residents as a house in multiple occupation, where those residents share one or more basic amenities. A seventh resident takes the property outside C4 and into sui generis, which means planning permission is needed for the use itself and no permitted change can deliver it.

That is the whole boundary. Everything else is about how you count, what evidence proves the existing use, and what the authority will condition.

What Class C4 covers

The class is drafted by reference to the statutory definition of a house in multiple occupation in the Housing Act 2004, excluding converted blocks of flats falling within the separate provision of that Act. The essential features are a single dwellinghouse, occupation by persons who do not form a single household, and shared use of a basic amenity such as a kitchen, bathroom or toilet.

Six is the ceiling on residents, not on bedrooms and not on households. A five bedroom house let to six people is C4. The same house let to seven is not.

Student lets, young professional shares and most small lodging arrangements fall within C4 where the numbers work. Care settings and children's homes are a different question entirely, and the C2 or C3 treatment of children's homes turns on the level of care provided rather than on resident numbers.

Where the seventh resident changes everything

A large house in multiple occupation with seven or more residents is sui generis. It sits outside the Use Classes Order, which has three consequences.

Planning permission is required for the change of use, and there is no permitted development route to it. The authority assesses the application on its merits against local plan policy, which in many areas includes a concentration policy limiting the proportion of HMOs within a given radius. And any permission granted will be for that specific use, so later changes need permission again.

The step from six to seven is therefore not incremental. It is a change in the regulatory category, and landlords who add a seventh room to improve yield frequently discover it through an enforcement enquiry rather than through advice.

Counting residents and households

A couple counts as one household but as two residents. Children count as residents. The household test decides whether the property is an HMO at all; the resident count decides whether it is C4 or sui generis. Confusing the two is the single most common error in this area.

Self-contained flats are not an HMO in the ordinary sense, because each flat is a separate dwelling, and converting a house into self-contained flats is a different change of use requiring permission in its own right.

A resident landlord complicates the count rather than removing it. The landlord living in the property is a resident for the purposes of the class, and in licensing terms a resident landlord plus four sharers is treated differently again under the housing legislation. Planning and licensing use overlapping but separate definitions, and a property can be licensable and lawful, licensable and unlawful, or neither.

The permitted change and Article 4 directions

The General Permitted Development Order 2015 permits change of use between Class C3 and Class C4 in both directions. In most of the country that means a family house can become a small HMO without planning permission.

Many authorities have made Article 4 directions removing the C3 to C4 right, typically across university districts and areas with high existing concentrations. Where a direction is in force, a full planning application is required for a small HMO, and the application will be assessed against the concentration policy.

Directions vary in their geography and their date of effect, and some cover a whole authority area while others are drawn tightly around a few wards. Check the authority's own list of directions and the mapped boundary, and check the date, because a use established before the direction took effect is unaffected by it.

Evidence of the lawful existing use

Buyers and lenders increasingly ask for proof that an existing HMO is lawful. A certificate of lawfulness for an existing use is the formal answer, and it is the only way to confirm that a use is lawful.

The evidence that works is documentary and dated. Tenancy agreements running continuously across the relevant period. Council tax records and bills. The HMO licence and its history. Utility accounts in tenants' names. Electoral roll entries. Letting agent records and advertisements. Sworn statements help but rarely succeed alone.

Enforcement time limits changed under the Levelling-up and Regeneration Act 2023, which moved the limit for breaches in England to ten years. The transitional position depends on when the breach occurred, so check the current position before assuming an older use is immune.

A Planning Statement supporting an application for a larger HMO should deal with the existing lawful use directly, because the planning balance looks different where the alternative is a lawful six person HMO rather than a family house.

Amenity standards and room sizes

Licensed HMOs are subject to nationally prescribed minimum sleeping room sizes: 6.51 m² for one person aged over ten, 10.22 m² for two persons aged over ten, and 4.64 m² for one child under ten. Rooms below 4.64 m² cannot be used as sleeping accommodation at all. Local authorities may set higher standards in their own licensing conditions.

Planning amenity standards sit alongside those and are often more demanding: usable kitchen and dining space proportionate to the number of occupants, adequate communal living space, cycle storage, bin storage sized for the collection regime, and usable external amenity space.

Bin storage is the item that most often attracts objection and condition. A seven person house generates a visible quantity of waste, and a scheme without a screened, accessible store at the front will be asked for one. Measure the store against the containers the authority actually issues, including recycling and food waste, rather than against a notional bin.

Parking, transport and conditions

Where the authority has a parking standard for HMOs, a Parking Survey using a recognised methodology is often the evidence that settles the highway objection, particularly in a controlled parking zone. On a larger scheme a Transport Statement scaled to the proposal covers access, servicing and cycle provision.

Expect conditions limiting the maximum number of residents, tying the use to the approved plans, requiring the bin and cycle stores to be provided before occupation, and in some areas requiring a management plan with a named contact. A condition capping occupancy at a stated number is normal on a sui generis permission and is worth proposing rather than resisting, because it gives the authority the control it needs to grant.

To see which documents an HMO application needs, enter your postcode on PlanScope.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.