Do I Need Planning Permission for a Rear Extension in London? (2026)

When is a rear extension permitted development?

A single-storey rear extension is permitted development when it stays within the standard depth limit for your house type and meets the other conditions — height no more than 4m, eaves no higher than 3m within 2m of a boundary, materials similar to the existing house, and no more than half the garden covered by buildings. The depth is always measured from the original rear wall of the house as it stood in 1948 or as first built, not from a wall added by a previous owner, which is a distinction that catches many homeowners out. Exceed any one condition and the whole extension needs an application — the conditions are cumulative, not a menu.

• 3m depth from the original rear wall for terraced and semi-detached houses
• 4m depth for detached houses
• Maximum height 4m; eaves within 2m of a boundary capped at 3m
• No more than 50% of the original garden/curtilage covered by buildings
• Similar materials to the existing house
• Not forward of the principal elevation onto a highway

What is the 6m 'larger home' extension and Prior Approval?

The Larger Home Extension scheme lets you go deeper — up to 6m for a terraced or semi-detached house and 8m for a detached house — without a full planning application, through a lighter Prior Approval process. You notify the council with a plan showing the depth and the neighbouring properties, and the council writes to the adjoining owners on both sides; if none objects within the 21-day consultation period, or the council rules the impact acceptable, you can build. It is faster and more certain than full planning, but it is not automatic — a neighbour objection brings the council's judgement on amenity into play, and the extension must still respect the 4m height cap and the 50% coverage rule. The right was made permanent in 2019 after years as a temporary measure, so it remains available in 2026. One practical point catches people out: you must not begin work until the council has either confirmed Prior Approval is not required or granted it, and building without waiting can invalidate the right entirely. The Prior Approval fee is lower than a full householder application, and because the council is only assessing the impact on the amenity of neighbouring properties — not the principle or the design — the decision is narrower and usually quicker, provided the notification plan clearly shows the depth, the height and the position of every adjoining property.

When does a rear extension need full planning permission?

Full planning permission is needed once the design or the location steps outside permitted development. A double-storey rear extension, a wraparound that includes a side return, anything over the depth and height limits, and any extension in a conservation area or on an Article 4 street all require a householder planning application. Flats and maisonettes have no extension PD rights at all and always need a full application, usually alongside freeholder consent. A householder application is decided against the council's local plan and design guidance, is publicised to neighbours, and takes a target eight weeks — longer if it goes to committee or attracts objections.

• Two-storey and wraparound rear extensions
• Anything exceeding the depth, height or coverage limits
• Conservation areas and Article 4 direction streets
• Flats and maisonettes — no PD rights

How is the 45-degree neighbour rule assessed?

Even a permitted-development extension is designed with neighbour daylight in mind, and for full applications the council applies a 45-degree test: a line drawn at 45 degrees in plan from the centre of a neighbour's nearest ground-floor window should not be crossed by your extension. Some boroughs also apply a 45-degree test in section, and the BRE daylight and sunlight guidance sits behind more detailed assessments. Extensions that breach these tests are the most common cause of objections and refusals. We design to the rule from the start and check the rear building line so the scheme the council sees is one it can approve.

How much does a rear extension add, and what will it cost to get consent?

Beyond the planning question, homeowners want to know whether the project is worth it and what the paperwork side costs. A well-designed single-storey rear extension typically adds both floor area and value to a London terrace, particularly when it delivers the open-plan kitchen-diner that buyers now expect, though the uplift depends heavily on the borough, the quality of the finish and how much garden you sacrifice.

Which route should I choose for my rear extension?

If your extension fits inside the standard depth limit and there is no Article 4 direction, the permitted-development route with a Lawful Development Certificate is fastest and lowest-risk. If you want the extra depth, the 6m Prior Approval route is the honest answer. If you are extending upward, wrapping around the side, in a conservation area, or own a flat, plan for full planning from day one. We assess which route your property actually qualifies for before any drawings are made, and prepare the building-regulations package the build will need regardless of the planning route. The sequence that works best is to settle the planning route first, because it dictates how much design freedom you have, then to develop the technical drawings that satisfy building control and let the builder price the job accurately. Trying to shortcut the planning question — building a 4m extension on a terrace 'because the neighbour did', for instance — is how homeowners end up with an enforcement notice or an unsellable house years later, when the depth turns out to have needed Prior Approval or full permission that was never obtained.