When Do You Need Planning Permission for an Extension?

Introduction

One of the most common questions homeowners ask before starting an extension project is: "Do I need planning permission?" The answer depends on the size and type of extension, the type of property, and whether any restrictions apply to your specific address.

This guide gives a plain-English overview of when planning permission is and isn't required for a house extension in England.

What Are Permitted Development Rights?

Permitted Development (PD) rights allow certain types of building work to be carried out without needing to apply for planning permission. They're set out in the Town and Country Planning (General Permitted Development) Order and apply across England — though they can be restricted or removed in certain areas.

Most single-storey rear extensions can be built under PD rights, subject to size limits. Double-storey extensions, first-floor extensions, and most side extensions require planning permission.

Single-Storey Rear Extensions

Under Permitted Development, you can add a single-storey rear extension without planning permission if it meets all of the following:

It doesn't exceed 3 metres in depth from the original rear wall (for terrace or semi-detached houses) or 4 metres (for detached houses)
It doesn't exceed 4 metres in height
Its roof pitch and materials are similar to the existing house
It's not forward of the principal elevation
The total area of all extensions (including any existing ones) doesn't exceed 50% of the original garden area

Larger Home Extension Scheme (prior approval): Extensions up to 6 metres (terrace/semi) or 8 metres (detached) in depth can be built under a prior approval process — you notify the council, they consult neighbours, and provide or refuse prior approval. This is not full planning permission but is a formal process.

Double-Storey Extensions

Double-storey rear extensions nearly always require planning permission. Permitted Development rights for double-storey extensions are more restricted — specifically:

They must not be within 7 metres of the rear boundary
They must not exceed the height of the existing roof
They must not be within 2 metres of the side boundary
The pitch must match the main house

In most London back garden situations, the 7-metre setback from the rear boundary rules out double-storey rear extensions under PD. These almost always need a planning application.

Side Extensions

Side extensions require planning permission in most cases. PD rights for side extensions exist but are limited:

The extension must not exceed half the width of the original house
It must not be forward of the principal elevation
There must be no first floor element above the side

In practice, most side extensions in London require a planning application. Where planning is needed for a side element and PD applies to the rear element, many clients apply for planning permission for the whole combined scheme.

When PD Rights Are Removed

PD rights can be removed or restricted in several circumstances:

Conservation Areas: In a conservation area, PD rights are restricted. Side extensions in a conservation area always require planning permission.
Article 4 Directions: Where a local council has issued an Article 4 Direction, it removes some or all PD rights in a specified area. Many London boroughs have Article 4 Directions covering parts of their residential areas.
Listed Buildings: Listed buildings have no PD rights. All external works require Listed Building Consent and usually planning permission.
Previous conditions: A condition on a previous planning permission may have removed PD rights for that specific property.

How to Check for Your Property

Before assuming PD rights apply:

Check whether your postcode is in a conservation area (your council's planning portal or local map layer will show this)
Check whether an Article 4 Direction applies to your area
Check previous planning permissions on your property for conditions that restrict PD
Consider applying for a Lawful Development Certificate — a formal confirmation from the council that your proposed works are lawful under PD rights. This provides documentary evidence useful when you sell the property.

Not sure whether your extension needs planning permission?
RCB Design & Build advises on planning position as part of our pre-construction process. Contact us.

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