Do I Need Planning Permission for a Rear Extension?

The short answer

A rear extension often does not need a full planning application. Many fall under permitted development (PD), a national set of rights that let you build within defined limits without applying for planning permission. But PD comes with conditions, and those rights can be restricted, so it's important to check before you build.

Permitted development size limits

For a single-storey rear extension, the standard depth limits are:

• 3m from the original rear wall for an attached house (terraced or semi-detached)
• 4m for a detached house

Under the Neighbour Consultation Scheme (a prior approval route), you may be able to go deeper:

• Up to 6m for an attached house
• Up to 8m for a detached house

"Original" means the house as first built, or as it stood in 1948 if older — not as you bought it. Previous extensions count against your allowance.

Height and other conditions

Even within depth limits, a single-storey rear extension must meet height rules to stay within PD:

• Maximum overall height of 4m
• Maximum eaves height of 3m if within 2m of a boundary
• It must not extend beyond the side of the original house or cover more than half the garden
• Materials should be similar in appearance to the existing house

PD rights are often removed on flats and maisonettes, in conservation areas, on listed buildings, and where an Article 4 direction applies. If any of these affect you, a full application is usually required.

The Neighbour Consultation Scheme

To use the larger 6m/8m allowances, you must apply for prior approval before starting work. The process runs like this:

• You notify the council with details and a plan
• The council contacts adjoining neighbours
• If a neighbour objects, the council assesses the impact on their amenity
• The council either grants or refuses prior approval

If no neighbour objects, approval is often straightforward. The statutory determination period for prior approval is set nationally, but processing times in practice vary — see /league-tables for how boroughs compare.

The 45-degree light test

Many councils apply a 45-degree rule to protect neighbours' daylight and outlook. A line is drawn at 45 degrees from the midpoint of the nearest habitable-room window on the neighbouring property. If your extension crosses that line, it may be judged to cause unacceptable loss of light.

This is guidance, not statute, and interpretation differs between authorities. Some also use a 25-degree vertical test. Check your borough's design guidance — practice varies, so see /boroughs for local detail.

Party Wall and building regulations

Even if you don't need planning permission, remember:

• Building regulations approval is almost always required for a habitable extension
• The Party Wall etc. Act 1996 may apply if you build near or on a shared boundary

These are separate from planning and run on their own processes.

Before you start

1. Confirm whether your PD rights are intact (check for Article 4 or conservation status)
2. Measure depth from the original rear wall
3. Decide whether you fall within standard PD or need prior approval
4. Consider a Lawful Development Certificate for certainty

This is general information, not legal advice. Rules and their local interpretation vary, so always confirm the position with your borough before starting work.