C4, Sui Generis and Section 257: a guide to HMO buy-to-let property types

Not all HMOs are the same

When people say “HMO”, they’re usually referring to a property that’s let to multiple tenants who share facilities. In practice, HMOs can be set up in different ways, and the planning and licensing classification can vary.

The key drivers are:

• Planning use class / planning status (how the property is treated under planning rules)
• How the building is configured (shared facilities vs self-contained units)
• How many people live there and whether they form separate households
• Whether the property is converted or purpose-built

Below is a clear overview of common HMO property types you may come across in buy-to-let HMO investing, including C4, Sui Generis, Section 257, and other categories such as purpose-built HMOs, bedsits, and mixed-use buildings.

HMO basics: what “HMO” means in property terms

In broad terms, an HMO is a property rented out to three or more tenants who are from two or more households, where basic amenities are shared (for example, kitchens and/or bathrooms).

However, the term “HMO” is sometimes used loosely in conversation. For investment and finance, what matters is the legal planning and licensing classification that applies to the specific building.

Use Class C4: small HMOs (the most common HMO category)

What C4 covers

Use Class C4 is commonly associated with smaller HMOs where tenants share facilities.

In many cases, C4 HMOs are the familiar “shared house” model: individual tenants rent rooms and share communal areas.

Planning considerations

Whether a conversion from a family home (often C3) to a C4 HMO is straightforward can depend on local planning controls. In some areas, certain conversions may be treated as permitted development; in others, additional local restrictions can require planning permission.

Licensing considerations

C4 HMOs are often where landlords first encounter HMO licensing requirements. Licensing can depend on the number of occupants, how many households are involved, and whether the local authority operates additional licensing schemes.

Mortgage considerations

C4 HMOs are often considered the easier end of the spectrum for HMO mortgage finance because they are more familiar to lenders and may be easier to assess. That said, lender criteria still vary by case.

Sui Generis: large HMOs and “in a class of their own”

What Sui Generis means

Sui Generis is a planning category used for certain larger or unusual uses. In the HMO context, it’s commonly associated with larger HMOs.

Planning considerations

Sui Generis HMOs typically require full planning permission. This is one reason why the building’s planning history and how it was originally authorised can be especially important.

Licensing considerations

Large HMOs are usually subject to more demanding licensing expectations, including closer scrutiny of management arrangements and fire safety.

Mortgage considerations

Sui Generis HMOs can narrow lender choice. Lenders may be more selective about:

• the number of bedrooms/occupants
• the building’s compliance history
• how the property is laid out and managed

Section 257 HMOs: self-contained flats inside a non-standard conversion

What a Section 257 HMO is

A Section 257 HMO is a legal category that generally relates to buildings converted into self-contained flats where the conversion did not meet modern building regulation standards.

Even though the flats are self-contained (each flat has its own facilities), the building can still fall within HMO rules because of the way it was converted and the standards that applied at the time.

Planning considerations

Planning treatment can differ from traditional shared-house HMOs because the units are self-contained. However, the building may still be subject to licensing and management requirements.

Licensing considerations

Whether a Section 257 property needs licensing can depend on the local authority approach and the specific circumstances of the building.

Mortgage considerations

Section 257 properties are often more difficult to finance. The main factor is usually the conversion compliance and building standards history, which can affect valuation and lender risk appetite.

Purpose-built HMOs: designed for multi-occupancy from the start

What “purpose-built” means

A purpose-built HMO is constructed (or developed) specifically for multi-occupancy use, rather than being converted from a typical family dwelling.

Examples can include modern developments with studio-style rooms and shared facilities, or purpose-designed accommodation blocks.

Planning considerations

Purpose-built HMOs generally require planning permission. Because the scheme is designed from the outset, it may align more closely with modern standards.

Licensing considerations

Purpose-built HMOs can still fall under the same licensing framework as other HMOs, depending on occupancy and whether any additional licensing schemes apply.

Mortgage considerations

Purpose-built HMOs can be more straightforward than older conversions because they may have clearer compliance documentation and modern fire safety and amenity arrangements. However, they can still be specialist finance, particularly where the development is complex.

Bedsits: room-by-room letting with shared or limited facilities

What a bedsit property is

A bedsit is typically a single room used for both sleeping and living, often with shared kitchen and/or bathroom facilities.

In many older buildings, bedsits are created by subdividing larger properties into individual letting rooms.

How bedsits differ from standard shared houses

The difference is usually about how tenants live day-to-day. Bedsits can feel more self-contained than a classic shared-house arrangement, but they can still be treated as HMOs for planning and licensing purposes.

Licensing considerations

Bedsit HMOs can require licensing depending on occupant numbers and whether the property meets the legal definition of an HMO.

Mortgage considerations

Some lenders are more comfortable with traditional shared-house layouts than bedsit configurations, particularly where communal space is limited. Lender appetite can vary, so the property’s layout and documentation matter.

Mixed-use HMOs: residential accommodation with a commercial element

What mixed-use means

A mixed-use property combines residential HMO accommodation with a non-residential element—commonly a shop, office, or other commercial unit at ground-floor level.

Planning considerations

Mixed-use buildings often involve change of use and can require full planning permission for the residential and/or commercial components.

Licensing considerations

The residential part may be subject to HMO licensing rules, while the commercial element is regulated under different frameworks.

Mortgage considerations

Mixed-use properties can significantly restrict lender choice. Many mainstream HMO mortgage products are aimed at purely residential buildings, so mixed-use may require specialist finance routes.

How HMO type can affect mortgage finance (in practice)

While every lender has its own criteria, HMO type can influence finance outcomes through common themes:

• Compliance and documentation: purpose-built and newer conversions may have clearer evidence of standards
• Planning history: whether the building’s use has been authorised correctly
• Fire safety and management complexity: larger and more unusual HMOs can be assessed more cautiously
• Valuation approach: some categories (particularly certain conversions) can be harder to value in a way that satisfies lender risk requirements

Quick comparison: common HMO types at a glance

Choosing the right HMO type for an investment strategy

The “best” HMO type isn’t just about yield. It’s also about how the property’s planning status, licensing position, and physical layout fit with the realities of mortgage finance and ongoing compliance.

For many investors, the starting point is to match the intended property type to:

• the local licensing environment
• the building’s compliance history
• the lender appetite for that specific category

Understanding which category you’re buying into—C4, Sui Generis, Section 257, purpose-built, bedsits, or mixed-use—helps you avoid surprises later in the process.