When Does an Extension Need Planning Permission?

In most cases, an extension falls under permitted development, but there are several limitations and size restrictions you should double-check.

To be eligible for permitted development, an extension must be:

  • No larger than 50% of the land available on your plot, based on the original property and excluding any previous extensions.
  • Away from the front of the house, or to the side if you live next to a road.
  • No higher than the tallest peak of your existing roof.
  • Four metres high as a maximum for a single-storey rear extension.
  • Built from similar materials to your property.
  • Without a balcony, raised deck or veranda.
  • Designed with privacy glazing if windows are facing the side of the plot, on an upper storey, or 1.7 metres from the floor.
  • Built no closer than seven metres from your rear boundary for two-storey extensions.

There are also rules about the size of the extension, depending on the type of home you have. Single-storey extensions cannot extend past your external wall more than eight metres if you live in a detached property or six metres for all other houses.

Extensions with one or more storeys cannot extend past the rear wall by more than three metres.

Conservatories are considered the same as extensions, so all these caveats apply if you’d like to build under permitted development.

The Pros and Cons of Building an Extension Without Planning Permission

It is doubtless preferable to have the correct approval in place before you start any building work because the time, costs and potential liabilities of constructing an illegal extension can be very stressful.

However, the majority of extensions are built with permitted development, and you can verify with a professional builder and the local planning office whether this applies to ensure you’re well within the rules.

What Is Retrospective Planning Permission?

If you mistakenly believe that your extension does not need permission or build an extension that breaches a local planning regulation, the best option is usually to apply for planning permission retrospectively.

That means you submit a full application based on an ‘as built’ construction, which you may either initiate yourself or need to do if the planning authority advises of a planning breach.

Why Might I Need Retrospective Planning Permission?

Common reasons for submitting a retrospective application include:

  • Not realising that planning permission was required.
  • Finding that the build needs to change or increasing the size of the extension due to a structural issue.
  • Wishing to proceed quickly and not having the time to wait for planning consent to arrive.

We recommend applying for a certificate of lawful development if you feel that your extension qualifies as a permitted development. If you decide to sell the property, you’ll find this document comes in handy, as it acts as formal evidence that your extension is lawful.

Implications of Building an Extension Without the Required Approval

As explained above, most councils will accept a retrospective planning application but don’t necessarily have to approve it – regardless of whether your extension has already been finished.

If your application is denied, the planning authorities might instruct you to reverse the work or face prosecution if you refuse to comply.

The local council may issue an enforcement notice that details what you need to do, whether to change the extension, knock it down, or adjust the construction to their satisfaction.

This can also happen if you have planning permission but haven’t met the conditions set by the planning office. They will deem your extension unlawful and treat it the same way they would a construction without permission at all.

You do have the right to appeal, so if you believe that an enforcement notice has been issued incorrectly, there is the potential to have this overturned.

Planning Permission Non-Compliance Fines

Most planning conflicts are resolved through a retrospective application process or adjusting the extension to ensure it meets the needs of the local council. They are far more likely to object if the building impacts public highways, pavements, pedestrian areas or access routes.

Therefore, you might need to replace clear glazing with obscured glass, move the boundary wall further away from your property perimeter, or reduce the height of the roof.

You can also resolve a planning breach by:

  • Appealing against the planning enforcement notice with proof that your extension is compliant or falls under permitted development rules.
  • Complying with the breach notice and making the requested changes.
  • Making a retrospective planning application.
  • Producing a certificate of lawful existing use.

In the worst-case scenario, you could be prosecuted if you have failed to apply for permission, had a retrospective application refused, or decided not to comply with an enforcement notice.

The fines associated with planning breaches are unlimited, and the case can be prosecuted as a criminal offence.

While highly unlikely and very unusual in an extension build, it is therefore essential you review the local planning rules and have the correct documentation in place before work begins.

Which Building Types Can I Construct Without Planning Permission?

Most outbuildings are permitted developments and won’t need formal planning permission. That includes smaller structures such as sheds, garden rooms and garages, as well as most moderately sized extensions.

We’ve listed the permitted development rules above. Still, it’s worth verifying these with a local contractor or the planning office since some areas have specific laws that vary from the national standards.

Mike Alexander
Hey there, I'm Mike - writer and part time home improvement expert at Refurbb. Since owning and refurbishing my own property in 2018, I've since been developing rental properties, writing about my home improvement endeavours, sharing what I've learned and connecting readers to reputable tradespeople in the UK.

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