Permitted development on a flat, maisonette or converted house

Flats have almost no permitted development rights. The householder rights that let a house owner build a rear extension, a dormer or a garden outbuilding sit in Part 1 of Schedule 2 to the Town and Country Planning (General Permitted Development) (England) Order 2015, and Part 1 applies only to a dwellinghouse. Article 2(1) of the Order says a dwellinghouse does not include a building containing one or more flats, or a flat contained within such a building. So a landlord who converts a house into two flats has removed Part 1 rights from the whole building, including the ground floor flat with its own garden.

That single definition explains most of the refusals and enforcement notices that land on conversion investors.

What the loss of Part 1 actually covers

Once a building contains flats, planning permission is needed for work that would be permitted at a house. In practice that means the rear extension, the side infill, the loft dormer, the roof light that projects, the porch, the hardstanding, the shed, the outbuilding, the chimney and the satellite dish are all applications rather than free rights.

It also applies to work on the flat itself. A ground floor flat owner adding a small conservatory is carrying out development on a building containing flats, and Part 1 Class A is not available to them regardless of who owns the garden.

There is no de minimis exemption that rescues small works. The test is whether the operation is development under section 55 of the Town and Country Planning Act 1990, and if it is, and no Class permits it, permission is required. Internal alterations that do not materially affect the external appearance are generally not development, which is why internal reconfiguration of a flat is usually a Building Regulations question rather than a planning one.

The horizontal division test, and why it matters

The GPDO defines a flat as a separate and self-contained set of premises forming part of a building from which it is divided horizontally. Read that carefully, because it decides the outcome.

A house divided vertically into two self-contained houses, each with its own front door at ground level and its own full height slice of the building, is two dwellinghouses. Each keeps Part 1 rights, subject to the usual limits measured against the original building. A house divided horizontally into an upper and a lower unit is two flats, and Part 1 is gone from both.

That is a genuine planning difference between two conversions that look similar on a floor plan. Before you buy a converted property for the extension potential, establish which division you are looking at.

Where a building has been converted to flats and then converted back to a single dwellinghouse, Part 1 rights become available again once the building is lawfully a single dwellinghouse. Lawfully is the operative word. If the reconversion itself needed permission and did not get it, the rights are not restored on paper.

A house in multiple occupation within Class C4 is a different case again. It is a house that has not been divided into flats, so Part 1 rights are generally still available, though the position for a large sui generis HMO is less settled and worth checking with the local planning authority before relying on it.

What a flat can still do without permission

Part 1 is not the only Part in Schedule 2, and several others apply to blocks of flats or to land generally.

• Gates, fences, walls and other means of enclosure under Part 2 Class A, at up to 1 m in height where adjacent to a highway used by vehicular traffic and up to 2 m elsewhere, with the usual exclusions around listed buildings.
• Painting the exterior of a building under Part 2 Class C, where it is not an advertisement.
• Electric vehicle charging outlets and upstands under Part 2, within the limits set in the relevant Class and subject to siting restrictions.
• Solar equipment on a block of flats and freestanding solar within the curtilage of a block of flats under Part 14, subject to the size, siting and conservation area limits in that Part.
• Air source heat pumps under Part 14 Class G, again subject to the limits and, where the relevant condition applies, the MCS planning standards.

Read the conditions attached to each Class rather than the headline. Part 14 in particular carries siting rules that bite hard on a flatted building with a shared roof, and several of those Classes exclude land within the curtilage of a listed building outright.

Adding storeys to a detached block under Part 20

Part 20 Class A is the one right that matters to a landlord with a purpose built block. It permits the construction of up to two additional storeys of new flats on a detached block of flats, within height limits and a date range for the original construction, subject to prior approval.

Prior approval is not a formality. The Class lists the matters the council must consider, which include external appearance, the amenity of existing and neighbouring premises with overlooking, privacy and loss of light, transport and highways, contamination, flooding and, for taller buildings, fire safety. The council can refuse on any of them.

Two practical points. The building must be detached, which rules out most Victorian conversion stock and end terraces. And a building reaching 18 m or seven storeys after the addition falls within the higher-risk building regime under the Building Safety Act 2022, which brings Planning Gateway One and a fire statement prepared by someone with the necessary fire engineering competence. That is a professional requirement, not a drafting exercise, and an AI-assisted draft is a starting point for that specialist rather than a substitute.

Part 20 also contains Class ZA, which allows the demolition of a single detached block of flats and its replacement with a new block or house, again through prior approval and again with strict qualifying criteria.

Conditions and directions that take away what is left

Even where a Class applies, two things can remove it.

A condition on the original planning permission can withdraw permitted development rights. Conditions of that kind are routine on new flatted schemes, particularly where amenity space or parking was tight at the time of approval. Read the permission for the building, not just the title deeds.

An Article 4 direction made by the council can withdraw specified Classes across a defined area. In conservation areas these commonly target front elevations, roof alterations and boundary treatment. Directions are published by the council and are recorded on planning.data.gov.uk, so there is no excuse for missing one.

Leasehold consents sit alongside all of this. Planning permission does not override a lease covenant, and a landlord's consent does not substitute for planning permission. They are separate systems and you need both.

Illustrative example: an investor buys the upper maisonette in a converted Edwardian semi and plans a rear dormer. The building contains flats, so Part 1 Class B is unavailable, and a full planning application is needed with a planning statement addressing amenity and the character of the roofscape. The neighbouring house, never converted, built the same dormer without an application two years earlier. Both readings are correct.

Getting certainty rather than assuming it

Where the answer turns on whether the division is horizontal or vertical, or on whether a previous reconversion was lawful, apply for a lawful development certificate. Section 192 of the Town and Country Planning Act 1990 covers proposed works and section 191 covers existing ones. A certificate is the only formal way to confirm that works are lawful, and it costs a fraction of an enforcement dispute.

This is general guidance rather than legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

Enter your postcode on PlanScope to see which supporting documents a flat conversion or upward extension is likely to need. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.