How to Check If You Need Planning Permission for a Home Extension in the UK

Before you order materials or book a builder, there is one question that can save you months of trouble: do you actually need planning permission for your extension? In England, many smaller extensions fall under permitted development rights, which means you can build without a full planning application, as long as your project stays within strict limits. Others need permission, and building without it can lead to enforcement action. This guide walks you through the checks in order, so you can find out where your project stands.

Start with the caveat that rules differ across the UK nations, and local details matter. This guide focuses on England, and flags where to check for Scotland, Wales and Northern Ireland. When in doubt, ask your local planning authority.

Step 1: Work out what kind of extension you are planning

Write down the basics before you search anything. What are you building: a single-storey rear extension, a side extension, a loft conversion, a two-storey extension, or a wraparound? How far will it extend from the original house, how tall will it be, and how close will it sit to your boundaries?

Those measurements decide almost everything. Permitted development limits are set in metres, and a project that is fine at three metres may need permission at four. Measure from the original house, not from a previous extension, because the rules use the house as it was first built, or as it stood on 1 July 1948 if it is older. If previous owners already extended, their work counts toward your limits.

Take photos and a rough sketch with measurements. You will need them for every later step.

Step 2: Check permitted development rules for your project type

Permitted development rights cover many common projects, but only within limits. For a single-storey rear extension on a detached house, for example, the extension can normally extend up to four metres from the original rear wall, while for other houses the limit is three metres, unless you use the larger home extension scheme, which allows up to eight metres for a detached house and six metres for other houses, subject to neighbour consultation.

Height matters too. A single-storey rear extension must normally be no higher than four metres, and if it sits within two metres of a boundary, its eaves height is limited to three metres. Side extensions have their own width and height limits, and extensions must generally use materials similar in appearance to the existing house.

These limits are a summary, not a substitute for the full rules. The Planning Portal website publishes the detailed permitted development guidance, and it is the best starting point. Read the section that matches your project type, and check each limit against your measurements, one by one.

Step 3: Check whether your home has lost its permitted development rights

Some homes have no permitted development rights at all, or only limited ones. This often happens with flats and maisonettes, which generally do not benefit from the same rights as houses. It also happens where a previous planning permission removed those rights through a condition, or where the home sits in an area with an Article 4 direction, which is a local rule that withdraws permitted development rights for certain types of work.

You can check this by looking at your property's planning history on your council's planning portal. Search by your address and read any past permissions and conditions. If you see a condition removing permitted development rights, assume you need permission and speak to the council.

Step 4: Check conservation areas, green belt and other designations

Location can change the answer even when the measurements look fine. In a conservation area, permitted development rights are more restricted, especially for extensions and cladding. In an Area of Outstanding Natural Beauty, now called a National Landscape, and in National Parks, limits are tighter again. Green belt land brings its own policy tests, and homes near a protected tree or in a flood risk area may face extra conditions.

Step 5: Use your council's planning portal and pre-application advice

Every council in England runs a planning portal where you can search applications, view policies and submit forms. Find yours by searching for your council name plus planning portal. Use it to check recent extensions on your street, because approved applications for similar homes are a useful reality check on what the council accepts.

If your project clearly falls within permitted development, you can apply for a Lawful Development Certificate. This is not planning permission, but it is formal written confirmation from the council that your project is lawful. Buyers and solicitors often ask for one when you sell, so it can save arguments later.

Step 6: Check building regulations separately

Contact your council's building control service, or a private approved inspector, early in the process. They will tell you what drawings and inspections you need. Keep the completion certificate safe, because you will need it when you sell.

Common mistakes to avoid

The most common mistake is measuring from the wrong point, such as from a previous extension rather than the original house. The second is assuming that a neighbour's extension means yours will be approved, when their property may have different rights, designations or history. The third is starting work on the strength of a builder's assurance alone. Builders know construction, but the legal question of permission is yours to get right.

Frequently Asked Questions

Do I always need planning permission for a home extension in the UK?

No. Many smaller extensions in England fall under permitted development rights and do not need a full planning application, as long as they stay within the size, height and location limits for that project type. Larger extensions, listed buildings and homes in designated areas often do need permission.

What is a Lawful Development Certificate?

It is a formal document from your council confirming that a proposed or existing project is lawful under permitted development rules. It is not planning permission, but it gives you written proof that can help when you sell your home or if a question arises later.

What happens if I build without planning permission when I needed it?

Your council can take enforcement action, which can require you to alter or remove the work. You can also apply for retrospective permission, but there is no guarantee it will be granted. Checking before you build is far safer than fixing the problem afterwards.