Do I Need Planning Permission for an Extension?

In many cases, you can build a house extension without planning permission, as long as it meets certain limits and conditions under permitted development rights. However, the rules are specific and vary based on the type of house, the size and location of the extension, and whether previous alterations have been made.

Planning permission is more likely to be required if your home is in a conservation area, National Park, Area of Outstanding Natural Beauty, or if it’s a listed building. It may also be needed if the proposed extension significantly alters the appearance of the property or impacts your neighbours.

When Is Planning Permission Not Required?

Most single-storey rear extensions are allowed under permitted development, provided they do not extend beyond three metres for terraced or semi-detached houses, or four metres for detached properties. You may also qualify for larger extensions under the Neighbour Consultation Scheme, which allows extensions up to six metres or eight metres respectively — but you must notify the council and your neighbours first.

The extension must not be taller than four metres, must not take up more than fifty percent of your garden space, and the materials used should be similar in appearance to the existing house. If the proposed extension is to the side of the house, it must be single-storey, no more than three metres high, and no wider than half the width of the original house.

When Do I Need Planning Permission for an Extension?

You’ll need full planning permission if the extension exceeds the permitted development size limits or if your property has had its permitted development rights removed — which is often the case with new-build homes, converted properties, or those under an Article 4 Direction.

Permission is also needed if the extension is two-storey and close to the boundary, changes the roofline, is forward of the principal elevation, or includes balconies or raised platforms. Any extension on a listed building will require listed building consent, regardless of size or scope.

Do I Need Building Regulations for an Extension?

Yes. Even if planning permission isn’t needed, building regulations approval is always required for extensions. This ensures the new structure meets legal standards for safety, energy efficiency, insulation, drainage, foundations, and structural stability. The plans will be reviewed by your local building control department or a private inspector, and inspections will take place throughout construction.

You’ll need to show compliance with rules around fire safety, ventilation, electrical wiring, glazing, and access. Once complete, a building control completion certificate will confirm the extension has been signed off legally.

What If I’m Extending a Flat or Maisonette?

Flats and maisonettes do not benefit from permitted development rights, so any extension will require full planning permission. Councils are particularly cautious with these types of properties due to shared boundaries, access issues, and impact on neighbouring residents. It’s important to speak to your local authority or a planning consultant early in the process to understand what’s likely to be approved.

Should I Apply for a Lawful Development Certificate?

If you're confident your extension is permitted development, it’s still worth applying for a Lawful Development Certificate (LDC). This is an optional but useful piece of documentation that proves your extension was legally constructed without the need for full planning permission. It’s especially helpful when selling or refinancing the property, as it gives buyers and lenders formal confirmation that no planning breach has occurred.

Do I Need Professional Advice Before Building an Extension?

While not legally required, professional advice from an architect, planning consultant, or structural engineer can save you time and prevent costly mistakes. They can ensure your plans meet all legal requirements, help you prepare the correct drawings for building control or planning applications, and guide you through the approval process.

If your property is in a sensitive area or has a complex history of development, getting expert input early on can be the difference between a smooth project and a planning headache.

Planning Permission and Permitted Development Are Not the Same Thing

People often confuse planning permission with permitted development rights. Just because your neighbour built something doesn’t mean you can. Permitted development is a set of nationally defined rules that only apply if certain conditions are met. These rights can be removed for individual homes or entire estates, especially new builds or properties in conservation areas.

Extensions Must Be "Proportionate"

Even under permitted development, extensions must be proportionate to the original house. Councils often reject designs that appear overbearing, out of scale with neighbouring homes, or result in a loss of character. Two homes may look similar but have very different outcomes depending on design quality, impact on neighbours, or local policy.

Neighbour Consultation Scheme Still Carries Risk

If you go through the Neighbour Consultation Scheme (for rear extensions between 3–6 metres on semis or 4–8 metres on detached homes), your project isn’t automatically approved. The council notifies neighbours, who can object. If valid objections are raised — for example, loss of light or privacy — the council can refuse the application, even if the extension is within size limits.

Previous Extensions Count Toward Your Allowance

Permitted development limits apply to the total enlargement of the property, not just the new part you’re building. If you (or a previous owner) have already extended the house, any new work may push you over the volume or footprint limit. You need to account for all existing extensions when calculating what’s still allowed.

There Are Tighter Rules for Side and Two-Storey Extensions

Side extensions are more restricted than rear ones. They must be single-storey, under 3 metres high, and no wider than half the width of the original house. Two-storey extensions always require planning permission and must not come within 7 metres of the rear boundary. They also face more scrutiny over overshadowing and impact on neighbours.

Front Extensions Are Almost Always Refused Without Permission

Building out from the front of your house almost always requires full planning permission. Councils are generally protective of street views and continuity of design, so front extensions are harder to get approved unless they’re small porches or infill sections that match the rest of the property.

Trees and Wildlife Can Be a Factor

If your proposed extension affects a protected tree (TPO) or is close to a site of ecological value, you may need to submit extra reports — like a tree survey or bat assessment — even if the extension is relatively modest. Skipping this step can result in delays or refusals.

Drainage and Flood Risk Aren’t Always Obvious

If your home is in a flood zone or has poor drainage capacity, the council may ask how surface water will be managed. You might need to include soakaways, permeable paving, or rainwater harvesting — even for small extensions. This is especially true in areas with clay soil or previous surface water issues.

A Lawful Development Certificate Is Worth It

Even if you’re confident your extension falls under permitted development, applying for a Lawful Development Certificate (LDC) protects you legally. It proves the extension was lawful at the time of construction, which is useful when selling, refinancing, or dealing with disputes. It’s not expensive and avoids uncertainty later.

Talk to Your Neighbours — Even If You Don’t Have To

You might not legally need to consult your neighbours, but it’s smart to do so anyway. Unhappy neighbours can raise objections, report potential breaches, or cause friction during the build. A quick heads-up and a look at the plans can go a long way in keeping things smooth.