Extending a flat or maisonette and why it differs

Extending a flat planning application starts from a harder position than the same work on a house. Part 1 of the General Permitted Development Order 2015, which gives householders the right to build rear extensions, side extensions, dormers and outbuildings without applying, applies to dwellinghouses, and the Order expressly excludes a building containing one or more flats and a flat contained within such a building. Every extension to a flat or maisonette therefore needs planning permission, however small.

Permission is also only half the problem. The lease, the freeholder and, in a converted house, the structure of the building below you all have to be dealt with separately, and none of them is resolved by a planning approval.

Why permitted development rights do not reach flats

The exclusion is deliberate. Householder rights were written for buildings in single ownership where the person building the extension owns the whole structure and the land around it. A flat sits inside a building somebody else may own, above and below other homes, so the same rights would let one occupier alter a shared structure.

A few permitted development rights do still reach blocks of flats, chiefly certain equipment installations, but nothing in the householder extension classes does. A maisonette with its own front door and its own garden is still a flat for these purposes if the building contains another dwelling.

Nor does the larger home extension prior approval route help, since that sits within the same Part 1. Applicants sometimes discover this after paying for drawings prepared on the assumption that the depth limits set out in our note on rear extension depth limits and the larger home right apply to them. They do not.

Ownership certificates and who has to be notified

This is the procedural trap that invalidates applications.

Section 65 of the Town and Country Planning Act 1990 and the Development Management Procedure Order 2015 require every application to carry an ownership certificate, and owner for this purpose means the freeholder or a lessee with at least seven years of the lease unexpired. A leaseholder applying to extend a flat is therefore rarely the only owner.

Where you do not own the whole application site, certificate B applies and notice has to be served on the other owners, with the date of service recorded on the certificate. In a converted house that usually means the freeholder. In a purpose built block it may also mean other long leaseholders whose demise includes part of the area affected, such as a shared garden.

Get the title entries before completing the form. Guessing at who owns what, and signing certificate A because it is simpler, makes the application invalid and can undermine a permission that is later granted.

Freeholder consent and the lease

Planning permission does not give you the right to build. The lease does, or it does not.

Most long leases demise the internal surfaces of the flat and reserve the structure, the external walls, the roof and the airspace to the freeholder. On that basis a rear extension built off the external wall, a dormer cut into the roof or a terrace formed over a back addition all involve work to something you do not own, and a licence to alter is needed.

Leases also carry covenants against alterations, against making openings in walls, and sometimes against any change to the external appearance. Read the actual lease rather than relying on what the managing agent says the standard lease contains.

Where the freeholder is a residents' management company, consent means persuading your neighbours, which takes longer than the planning application. Start that conversation before you spend money on drawings.

The Party Wall etc. Act 1996 runs alongside all of this where the work affects a party structure, and it applies between flats stacked vertically as well as between houses side by side.

Structure in a converted house

Converted houses were not designed as flats, and the structure reflects that. Timber floors spanning between loadbearing walls, shallow Victorian foundations, and chimney breasts that have been removed at one level and not another are all common.

An extension to an upper flat has to land its loads somewhere. That usually means new structure through the flat below, which requires access, consent from that leaseholder, and a design that does not leave a steel beam in the middle of somebody's living room.

Get a structural engineer to look at the building before the design is fixed, not after permission. A scheme that is unbuildable without work inside a neighbour's home is a scheme that will not proceed, whatever the decision notice says. The same logic applies to lower ground floor work, where the considerations set out in our note on basement extensions under an existing house apply with the added complication of shared ownership.

Effects on the flats above and below

Planning officers assess amenity for the occupiers of the same building as carefully as for the neighbours next door, and in flatted buildings the relationships are tighter.

A rear extension at ground floor level takes light from the window of the flat above it if it is deep, and it creates a flat roof that the upper flat may overlook or, worse, may claim as a terrace. A dormer alters the roof profile and can affect the ceiling line of the flat below where the structure is shared. A new terrace over a back addition overlooks the gardens of the whole terrace, and that is the single most common refusal reason for maisonette schemes.

The 45 degree test used for daylight to neighbouring windows applies vertically within a building as well as horizontally between them, and the method is set out in our note on the 45 degree rule and a rear extension.

Noise transfer is a separate point. Converting a rear extension into a kitchen directly under a bedroom invites an objection that officers take seriously.

Where the block is tall

Buildings at or above 18 m or seven storeys fall within the higher-risk regime, and building work to them is subject to the building control route run by the Building Safety Regulator rather than by the local authority or an approved inspector.

That has programme consequences. Approval before work starts takes considerably longer than a conventional building control application, and changes during construction have to go back through the process.

It rarely affects a single flat extension in a low rise building. It does affect balcony additions, additional storeys and facade alterations on tall blocks, and it should be checked at feasibility stage rather than discovered later.

What to submit

A flat extension application is a full householder-style application with a few additions.

1. The application form with the correct ownership certificate and, where certificate B applies, the date notice was served on each owner.
2. Existing and proposed plans, elevations and sections at a stated scale, with a site location plan.
3. A short Planning Statement setting the scheme against the adopted plan's amenity and design policies, and dealing expressly with effects on the other flats in the building.
4. A Design and Access Statement where the building is in a conservation area or a World Heritage Site and the scheme creates a dwelling or adds 100 m2 or more of floorspace.
5. A Heritage Statement where the building is listed or in a conservation area, with listed building consent applied for separately where relevant.

Dealing with the other occupiers openly in the statement helps. Officers receive objections from within the building on most of these applications, and an application that has already addressed the internal relationships reads better than one that treats the building as though it were a house.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

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