Single-Storey Rear Extensions: Design, Size Limits and Planning

A single-storey rear extension is the most common way to add ground-floor space to a London home — a bigger kitchen, a dining area, or a light-filled family room opening onto the garden. This guide explains how far you can build, when you need planning permission, and how to get the design right.

How big can a single-storey rear extension be?

Under permitted development, a single-storey rear extension can normally project up to 3m beyond the original rear wall for a terraced or semi-detached house, or 4m for a detached house, with a maximum height of 4m. Within 2m of a boundary, the eaves must not exceed 3m.

Larger projects — up to 6m (terraced/semi) or 8m (detached) — may be possible under the Larger Home Extension prior-approval route, which involves a neighbour consultation scheme run by your council. These allowances do not apply in conservation areas, and permitted development can be removed altogether by an Article 4 direction, so always confirm the current position for your property with the Planning Portal or your local authority before relying on it.

When do you need planning permission?

You will generally need full planning permission if the extension exceeds permitted development limits, if you live in a conservation area or a listed building, or if permitted development rights have been removed. Even where permission is not required, obtaining a Lawful Development Certificate is worthwhile — it proves the extension was lawful when you come to sell. Building Regulations approval applies in every case.

Designing a single-storey rear extension

• Light the middle of the plan: roof lights or a lantern stop the original room behind the extension becoming dark.

• Think about the whole ground floor: an extension should improve how the existing rooms work, not just bolt space on the back.

• Choose the roof carefully: flat roofs with roof lights, or a pitched roof with glazing, each change the feel and the cost.

• Get the threshold right: a level threshold to the garden makes the room feel twice the size in summer.

Terraced, semi-detached or detached

Each house type brings its own constraints — party walls and side returns on terraces, boundary distances on semis, more freedom (and often more scope) on detached homes. A design that ignores these tends to hit problems at planning or on site.

Party Wall matters

If your extension affects a shared wall or is built close to a boundary, the Party Wall etc. Act 1996 is likely to apply and you will need to serve notice on neighbours. Handling this early and properly avoids delays and disputes once work starts.

How Studio MASS can help

We design single-storey rear extensions across London, from concept through planning or lawful-development approval to technical drawings and builder appointment. Book a free consultation: email info@studio-mass.com, call 0208 638 6727, or use the Talk to Us button above.

Last reviewed: July 2026. Permitted development limits change from time to time — confirm the current rules before you rely on them.

About the author

Robert McBride BSc(Hons) MFPWS is an architectural technologist and director of Studio MASS, a London practice specialising in residential extensions, conversions and new-build homes.