Extensions to dwellinghouses are one of the most frequently built types of residential development in England. The permitted development rights that apply to houses allow a considerable range of extensions to be built without a planning application, but the rules are specific and misunderstanding them can result in work being carried out without the required consent.

Permitted Development for Extensions

Under the General Permitted Development Order, certain extensions to dwellinghouses are permitted development and do not require planning permission. The principal rules for single-storey rear extensions on a detached house allow an extension up to eight metres deep beyond the original rear wall, subject to a maximum eaves height of four metres and a maximum overall height of four metres. For semi-detached and terraced houses, the permitted development depth limit for a single-storey rear extension is six metres.

These deeper limits, introduced as a temporary measure and subsequently made permanent, apply under the prior approval procedure. For extensions up to four metres deep on a detached house or three metres deep on any other house, no prior approval is required and the extension is straightforwardly permitted development provided the other conditions are met.

Conditions That Must Be Met

To be permitted development, an extension must satisfy several conditions. It must not extend beyond the side elevation of the original house unless it is a rear extension. Side extensions require planning permission in conservation areas and are restricted to single-storey with a maximum width of half the width of the original house. The extension must not be taller than the existing roofline. In designated areas such as National Parks and conservation areas, the rules are more restrictive and many rear extensions require planning permission. Cladding the exterior in a material different from the existing house requires planning permission in conservation areas. The total footprint of all extensions and outbuildings must not exceed 50 percent of the curtilage of the original house.

Two-Storey and First-Floor Extensions

Two-storey extensions and extensions that involve a first floor or roof alteration are generally more restricted under permitted development. A two-storey rear extension is limited to three metres beyond the original rear wall and must not be within seven metres of the rear boundary. These extensions also require prior approval in some cases. First-floor side extensions require planning permission in most circumstances.

When Planning Permission Is Required

Planning permission is required for extensions that exceed the permitted development limits, for any extension to a flat or maisonette, for extensions in designated areas that exceed the more restrictive limits applicable there, for extensions to listed buildings, and for any extension that involves a material change of use of any part of the building.

Building Regulations

Even where planning permission is not required, Building Regulations approval is needed for virtually all extensions. This covers the structural design, thermal performance, drainage, electrical work, fire safety, and other technical aspects of the extension. Building Regulations approval must be obtained before or during the work, and a completion certificate issued on satisfactory final inspection. This certificate is required when you come to sell the property.

Summary

Many single-storey rear extensions to houses fall within permitted development rights and do not require planning permission, subject to size limits and conditions. Two-storey extensions, side extensions, extensions in designated areas, and extensions to listed buildings or flats generally require planning permission. All extensions require Building Regulations approval regardless of their planning status. A Lawful Development Certificate provides formal written confirmation that an extension is lawful under permitted development, which is a useful document when selling the property.