Rear extensions are one of the most popular home improvements in the UK, adding space, light and value. But before you get excited about bi-fold doors and roof lanterns, you need to know whether your project needs planning permission. Most rear extensions do require approval unless they fall within permitted development rules. The key is to check your local limits before you hire a designer, so you don't waste money on drawings that can't be built.

When planning permission is likely to be needed

If your proposed extension stays within the permitted development (PD) limits, you can often build without a full planning application. However, many rear extensions exceed those limits – perhaps you want more depth, a higher roof, or a two-storey design. You will also need planning permission if your property is a flat or maisonette, a listed building, or if your PD rights have been removed by an Article 4 direction. Conservation areas and areas of outstanding natural beauty have extra restrictions, too. So the first step is always to check with your local planning authority, not assume.

The permitted development limits for rear extensions

For a single-storey rear extension, the rules are relatively generous but specific. Here are the main limits you must respect:

• Depth: up to 3 metres beyond the rear wall for terraced and semi-detached houses, or up to 4 metres for detached houses.
• Height: the extension must not be higher than 4 metres, and the eaves must not exceed 3 metres if the extension is within 2 metres of your boundary.
• Materials: they should match the existing house as closely as possible – brick, tiles, render, etc.
• Coverage: the extension must not cover more than half of the land around the original house.
• Design: no balconies, verandas, raised platforms or chimneys. Roof lines must not extend beyond the highest part of the existing roof.

Two-storey rear extensions are much more restricted. Under PD, they can only extend up to 3 metres, and the height limits (eaves 3 metres, overall 4 metres) mean most two-storey designs will need full planning permission. There is also a neighbour consultation scheme that allows larger single-storey extensions – up to 6 metres for terraced and semi-detached, or 8 metres for detached – but you must apply for prior approval first, and your neighbours will be consulted.

When permitted development rights are removed

Even if your extension would normally be PD, you may still need permission if your property has had its PD rights removed. This happens through an Article 4 direction, which councils use to protect local character. You will also need permission if you live in a conservation area (for certain works), a listed building, or a flat. If you have already extended your home, your remaining PD allowance may be reduced. And if you are in a new build, PD rights are often removed for a period. Always check the planning history of your property and any local designations before assuming you can build.

How to check your local limits before hiring a designer

Doing your homework now saves time, money and stress. Follow these practical steps:

• Use the Planning Portal's interactive guide to see if your idea falls under PD – it's free and quick.
• Visit your local council's website and search for Article 4 directions, conservation area maps and local plan policies.
• Look at your property's planning history – any previous extensions or conditions may affect your rights.
• Request a lawful development certificate (LDC) from the council. This is a formal confirmation that your project is PD, and it's invaluable if you later sell your home.
• Consider pre-application advice from your council. For a small fee, you can get written feedback on your proposal before you submit a full application.
• Talk to a planning consultant if your situation is complex – for example, if you are in a conservation area or have a listed building.

Only after you have confirmed the limits should you hire an architect or designer. Give them a clear brief based on what is actually allowed.

The planning application process and common pitfalls

If you need full planning permission, expect the process to take around 8 weeks for a householder application, though it can be longer. You'll need scaled drawings, a site plan, and often a design and access statement. Fees vary by council but are typically a few hundred pounds. Once approved, you must build exactly what was granted – any changes need a new application.

Common pitfalls to avoid include:

• Overlooking neighbours – windows and balconies that invade privacy are a frequent reason for refusal.
• Loss of light – your extension should not leave neighbours in darkness.
• Not matching materials – this can lead to enforcement action even if you have permission.
• Ignoring trees – protected trees (TPOs) need consent, and roots can affect foundations.
• Forgetting building regulations – these are separate from planning and cover structure, insulation, drainage and fire safety.
• Starting work before permission – this is a criminal offence and can lead to enforcement notices.
• Party wall issues – if your extension is near a boundary, you may need a Party Wall Agreement.

Get the planning permission sorted first, then enjoy the build. A little preparation now will save a lot of heartache later.