Extending a flat: why householder rights do not apply

A flat has no permitted development rights, so flat extension planning permission is needed for almost anything that changes its footprint, its roof or its external walls. Schedule 2 Part 1 of the Town and Country Planning (General Permitted Development) (England) Order 2015 grants rights to a dwellinghouse, and article 2(1) of the same Order states that a dwellinghouse does not include a building containing one or more flats, or a flat contained within such a building. Maisonettes are caught by the identical wording. The practical result is that work a neighbour in a house could build without asking anyone will need a full application from you, with drawings, a fee and an eight week wait.

That single definition drives everything else below.

Why Part 1 stops at the door of a flat

Part 1 covers the classes householders recognise: Class A for enlargement, improvement or other alteration, Class B for roof additions, Class C for other roof alterations, Class D for porches and Class E for outbuildings within the curtilage. Every one of them is expressed as a right enjoyed by "the dwellinghouse". Strip out the buildings that article 2(1) excludes and flats disappear from the whole Part.

There is no partial version of this. A ground floor flat with its own front door, its own garden and its own council tax band still sits inside a building containing flats, so Class A does not reach it. Neither does Class E, which is why a garden room behind a converted Victorian terrace needs permission when the identical structure behind an unconverted one does not.

Two narrower rights do exist elsewhere in the Order. Part 20 allows additional storeys on certain blocks of flats through prior approval, but that route creates new dwellings for the building owner rather than enlarging any existing flat. Part 14 covers domestic microgeneration, and parts of it apply to flats, which is how a solar array can sometimes go on a block's roof without a full application. Neither helps a leaseholder who wants a rear extension.

Maisonettes, converted houses and where the line falls

A maisonette is a flat arranged over two storeys. The arrangement makes no difference to article 2(1), because the building still contains more than one dwelling. Owners often assume otherwise, partly because maisonettes look like houses from the street and partly because estate agents describe them loosely.

The line moves only when the building stops containing flats. If a house was converted into two flats and one owner has since bought and physically reunited both, the building may have reverted to a single dwellinghouse. That is a question of fact and planning history, not of intention, and the safe way to settle it is an application for a lawful development certificate for an existing use under section 191 of the Town and Country Planning Act 1990. A certificate is the only formal way to confirm that works or a use are lawful, and it is worth the fee before anyone orders steelwork.

Watch for the reverse trap too. Creating a self-contained flat inside a house is a material change of use under section 55(3)(a) of the 1990 Act and needs permission on its own account, whatever the building works involve.

What the application has to contain

Work to a flat cannot go in on the householder application form. That form is reserved for works to a house and its curtilage, so alterations to a flat are submitted as a full application for planning permission, with the fee band that goes with it. Fees are set by regulations and indexed annually, so check the current schedule on the Planning Portal before you budget.

The change of form has two consequences people notice late. Full applications attract a 21 day publicity period and the ordinary appeal route rather than the 12 week householder appeal service. And where a council asks for supporting text, a short Planning Statement that sets the proposal against the adopted local plan carries more weight than a covering letter, because the officer can lift the policy assessment straight into the report.

A workable submission for a modest flat extension usually runs:

1. Location plan at 1:1250 with the application site edged red and any other land in the same ownership edged blue.
2. Existing and proposed plans and elevations at 1:100 or 1:50, with a section through the extension.
3. A completed ownership certificate, which for a leaseholder means certificate B and formal notice on the freeholder.
4. A Design and Access Statement if the site is in a conservation area, otherwise a short design note.
5. Photographs of the rear elevation and the boundary treatments either side.

Certificate B matters more than its size suggests. Serving notice on the freeholder is a statutory step, and getting it wrong is a common reason for a council to hold an application at validation.

Consents that run alongside the planning application

Planning permission settles the public interest in the proposal. It settles nothing about your right to build.

Almost every long lease reserves the external structure and often the roof, the airspace and the garden to the freeholder, and requires a licence for alterations before work starts. That licence is a contractual matter, usually involving the freeholder's surveyor and a fee, and it can take longer than the planning application. Start it in parallel, not afterwards.

The Party Wall etc. Act 1996 applies as well. Cutting into the party wall between two flats, or excavating within three metres of an adjoining owner's structure and deeper than its foundations, triggers a notice with its own periods and its own award procedure. Neighbouring leaseholders and the freeholder can all be adjoining owners for these purposes, so the notice list is longer in a converted house than in a semi. A schedule of condition agreed before work starts is the least costly protection either side will buy.

Amenity for the flat below and the neighbours opposite

Officers assessing a flat extension think about the other dwellings in the same building first. A rear extension at ground floor level takes light from the lower ground flat's rear window and often sits directly beneath a first floor bedroom. Where a numerical test applies, most councils use the 45 degree rule in plan and section, taken from the centre of the neighbouring window, and some householder design guides set a stricter line for rear additions in terraces.

Overlooking is assessed between habitable room windows, and many local design guides use a back to back separation of around 21 metres as a starting point rather than a rule. Obscure glazing and high level cills answer a side facing window. They rarely answer a new opening straight down a light well.

Noise transfer is a planning matter where the works change the relationship between dwellings, for instance a new hard floor above a bedroom. Building Regulations Part E sets the sound insulation standard, and the two regimes run separately.

Roof terraces and the objections they attract

Roof terraces are the element officers scrutinise hardest in a flat extension scheme, and they would be excluded from Class A even on a house, because the class does not permit a veranda, balcony or raised platform. On a flat the question is simply whether the amenity case holds up.

Three points decide most of them. The first is the outlook from the terrace: if the parapet gives a clear view into a neighbouring rear window or garden at close range, a condition will not rescue it. The second is the screening, where a 1.7 metre or 1.8 metre obscure screen on the flank is normal and an officer will want it fixed by condition and shown on the drawings. The third is use, since councils increasingly condition against amplified music and against use after a set hour where the terrace sits above other homes.

An illustrative example. A first floor flat in a converted 1930s semi proposes a rear terrace over an existing back addition, five metres from the boundary and overlooking two gardens. Raising the screen to 1.8 metres on both flanks, setting the terrace back a metre from the rear edge and keeping the parapet solid rather than glazed turns a scheme that reads as a viewing platform into one an officer can defend. The drawings, not the covering letter, have to show all three.

If you are working out which documents your own building will need, enter your postcode on PlanScope to see which of the 66 report types the site is likely to trigger.

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.