The Complete Guide to House Extension Planning Permission in London

The Complete Guide to House Extension Planning Permission in London

Introduction

Planning permission is one of the first questions homeowners in London ask when they start thinking about an extension. The rules can seem complicated — especially in a city where many properties sit in conservation areas, Article 4 direction zones, or have specific restrictions attached to their planning history.

The good news is that many common extensions in London can proceed without a planning application under permitted development rights. The challenge is knowing exactly when this applies, and when it doesn't.

This guide gives you a clear framework for understanding what's possible, what requires permission, and what to check before you start.

What Is Permitted Development?

Permitted development (PD) rights allow certain types of building work to proceed without a full planning application, provided the work stays within defined limits. These rights are granted automatically under the Town and Country Planning (General Permitted Development) (England) Order 2015 and subsequent amendments.

For houses (not flats or maisonettes), permitted development rights cover a wide range of work — including rear extensions, loft conversions, and certain outbuildings — subject to specific size and design conditions.

Important: PD rights apply to houses only, not to flats or apartments. If you own a flat, any external alterations will almost certainly require a planning application.

Rear Extensions Under Permitted Development

For a single-storey rear extension, permitted development rights allow:

• Up to 3 metres beyond the original rear wall for a terrace or semi-detached house
• Up to 4 metres for a detached house
• A maximum eaves height of 3 metres (and no higher than the existing eaves)
• A maximum overall height of 4 metres
• The extension must not cover more than half the garden area

Under the Larger Home Extension Scheme (formerly the Larger Homes prior approval process), single-storey rear extensions can be extended to:

• Up to 6 metres for terraced or semi-detached houses
• Up to 8 metres for detached houses

This larger scheme requires a prior approval application to the local authority before construction begins. Neighbours are notified, and the local authority must confirm whether prior approval is required and whether it is granted.

Side Extensions

Side extensions are generally more restricted under permitted development:

• Maximum of 50% of the width of the original house
• Single storey only (the first 4 metres from the original rear wall can be double-height under some conditions, but side extensions to two storeys almost always require full planning permission)
• Not closer to the highway than any part of the original house
• Not forward of the principal elevation

Side extensions in many parts of London also fall within Article 4 directions (explained below), which remove permitted development rights entirely.

Double-Storey Extensions

Double-storey rear extensions are more restricted under permitted development:

• No closer than 7 metres to the rear boundary
• Maximum height cannot exceed the existing roof ridge
• The roof pitch must match the main house where possible
• Upper storey windows on the side elevation must be obscure-glazed and non-opening (below 1.7 metres)

In practice, many double-storey extensions in London require full planning permission, particularly on smaller plots or where the rear garden does not provide enough depth to meet the 7-metre rule.

What Removes Permitted Development Rights?

Several circumstances mean permitted development rights don't apply, even if the proposed extension would normally qualify:

Article 4 Directions. Local planning authorities can remove permitted development rights in specific areas using an Article 4 Direction. This is common in conservation areas, around listed buildings, and in some residential neighbourhoods where the local authority wants to retain design control. If your property sits within an Article 4 area, almost all external alterations — including extensions that would otherwise be permitted — require a planning application.

Previous extensions. If the property has already been extended under permitted development, those extensions count against the total permitted. The limits apply to the entire extension of the house beyond its original footprint, not just the new work being proposed.

Conditions on the original planning permission. Some properties were built or approved with conditions that remove or restrict permitted development rights. These will appear on the original planning permission documentation.

Flats and maisonettes. As noted above, PD rights do not apply. All external work requires planning permission.

How to Check Whether You Need Planning Permission

The most reliable way to check is to:

1. Check whether your property is in a conservation area or Article 4 zone. Your local authority's planning portal will show this.
2. Review the original planning history for the property. Check whether any existing conditions restrict PD rights.
3. Submit a Certificate of Lawful Development (CLD) application. This is a formal written confirmation from the local authority that a proposed extension is lawful under permitted development. It provides legal protection and is often required by mortgage lenders at point of sale.
4. Speak to a planning consultant or design-and-build contractor who understands the local borough's approach.

At RCB Design & Build, we help clients understand what's achievable under permitted development and what requires a planning application before any design work begins. This ensures the design process is aligned with what's actually approvable.

Thinking about extending your home in London?
Contact RCB Design & Build to discuss your project and get clarity on what's possible before you invest in architectural drawings.

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