What Size Extension Can I Build Without Permission

What Size Extension Can I Build Without Permission

One of the most common questions homeowners ask when planning to extend their property is how big an extension can be built without needing planning permission. The answer depends on permitted development rights, which allow certain works to be carried out without a full planning application. These rights set strict limits on size, height and location, and they apply only to houses, not to flats, maisonettes or listed buildings.

Rear Extensions Under Permitted Development

Single storey rear extensions are often allowed without planning permission, provided they stay within size limits. For a terraced or semi-detached house, you can extend up to three metres from the original rear wall. For a detached house, this increases to four metres. Under the larger home extension scheme, this can be doubled to six metres for semis and eight metres for detached homes, but only if you go through the prior approval process, which involves notifying the council and allowing neighbours to comment.

Two Storey Rear Extensions

Permitted development also allows for two storey rear extensions, but the rules are much stricter. The extension can project no more than three metres beyond the original rear wall, and the boundary to the rear must be at least seven metres away. The roof pitch must match the main house, and the extension must not be taller than the existing property. Any design that exceeds these limits requires planning permission.

Side Extensions

Single storey side extensions are possible under permitted development, but they must be modest in scale. They cannot be higher than four metres and must not be wider than half the width of the original house. Materials used must be similar in appearance to the main property. Two storey side extensions are not normally permitted under permitted development, so a planning application will almost always be required.

Height and Volume Restrictions

All extensions under permitted development must remain within height and volume limits. The eaves height cannot exceed three metres if within two metres of a boundary, and the overall height cannot be greater than the ridge height of the original house. Loft and roof extensions have additional volume limits of up to 40 cubic metres for terraced houses and 50 cubic metres for detached and semi-detached homes.

Protected Areas and Restrictions

Permitted development rights can be limited or removed in certain areas. If your home is in a conservation area, national park, area of outstanding natural beauty or subject to an Article 4 Direction, stricter rules will apply and you may need planning permission even for small extensions. Listed buildings always require listed building consent for extensions, regardless of size.

Building Regulations

Even if your extension does not need planning permission, it will still need building regulations approval. This ensures the structure is safe, properly insulated, and meets standards for fire safety, ventilation and drainage. Building control officers will inspect the work at key stages, and a completion certificate will be issued once the project is finished.

Summary

The size of extension you can build without planning permission in the UK depends on permitted development rights. Single storey rear extensions of up to three or four metres are usually allowed, with larger projects possible under the prior approval process. Two storey extensions and side extensions have stricter limits, and special rules apply in protected areas. Building regulations approval is always required, regardless of planning status. By checking the permitted development rules carefully, you can plan your extension with confidence and avoid unnecessary delays.