Do I need planning permission for a rear extension in London?

The short answer

Many rear extensions in London are built without a planning application, under permitted development rights. Whether yours qualifies depends on four things: the depth you project into the garden, the height, how close you build to the boundary, and whether your property carries a designation that removes those rights in the first place.

Permitted development is not a loophole or a shortcut. It is a national grant of planning permission, set out in the General Permitted Development Order, that allows certain householder works without a formal application. If your proposal sits inside the limits, you do not need permission. If it steps outside them by a single measurement, you do.

The depth limits that decide most projects

For a single-storey rear extension, the depth you can project beyond the original rear wall is the figure that settles most schemes.

A terraced or semi-detached house may project 3 metres. A detached house may project 4 metres. Beyond that, and up to 6 metres for a terrace or semi and 8 metres for a detached house, the work may still avoid a full application, but only through the Larger Home Extension route, which requires prior approval and a neighbour consultation exercise run by the council.

The word original matters more than homeowners expect. It means the house as it stood on 1 July 1948, or as first built if later. A previous owner's small rear addition in the 1980s counts against your allowance, even though you did not build it and it may not appear on any drawing you hold.

Height, boundaries and the rest of the tests

Depth is necessary but not sufficient. A single-storey rear extension must also sit within a maximum height of 4 metres. Where any part of it stands within 2 metres of a boundary, the eaves height is capped at 3 metres, which is the test that most often forces a redesign of a roof form on a narrow London plot.

Two further limits apply across the whole property rather than to the extension alone. No more than half the area of land around the original house may be covered by buildings, which catches houses that have already added a garden room or a large outbuilding. And the extension must not project forward of the principal elevation fronting a highway.

Two-storey rear extensions are governed more tightly again: a maximum projection of 3 metres, and they cannot be built within 7 metres of the rear boundary. On a typical London terrace with a short garden, that second test rules out a great many two-storey schemes before the design begins.

What removes your rights entirely

Three designations change the answer regardless of dimensions.

A listed building requires listed building consent for works affecting its character, and that runs alongside, not instead of, any planning requirement. Internal alterations are caught too, which surprises many owners.

A conservation area restricts permitted development, removing side extensions and cladding among other things, and adds design scrutiny to what remains.

An Article 4 direction is a decision by your council to withdraw specified permitted development rights across a defined area, often a single street or a group of streets. It is invisible from the pavement and does not appear on your deeds. Where one is in place, work that would be permitted two streets away needs a full application on your address.

Flats and maisonettes have no householder permitted development rights at all. If you live in one, the question does not arise: an application is required.

Why a certificate is still worth having

Permitted development means no application is required, not that you can prove it later. A Lawful Development Certificate is a formal confirmation from the council that the works were lawful. It is not compulsory.

It becomes valuable at the point of sale. A buyer's solicitor will ask what consent covered the extension, and an absence of paperwork is a familiar cause of delay, retained funds or a renegotiated price. The certificate closes that question permanently, and costs a fraction of what an indemnity policy and a stalled sale will cost you later.

Permitted development is not the same as no consent

Building regulations apply whether or not planning permission is needed. Structural openings, foundations, insulation, drainage, fire escape and ventilation all have to be signed off, and the approval runs on a separate track from planning.

Neighbouring party structures bring the Party Wall etc. Act 1996 into play, which is a further, separate process with its own notice periods. Neither of these is discretionary, and neither is affected by whether your scheme is permitted development.

How to establish where you stand

Start by confirming the designations on your address rather than the dimensions of your scheme, because a designation can render the measurements irrelevant. Check whether the property is listed, whether it sits in a conservation area, and whether an Article 4 direction applies.

Then establish the original footprint, not the current one, and measure what has already been added.

Only then is it worth testing depth, height and boundary distances against the limits. Designed the other way round, you can spend weeks refining a scheme that was never available to you.