Permitted Development Rules for Extensions

The first thing to think about is how big your extension will be. If you want to build under permitted development, the new structure cannot be over 50% of the land on your property, based on the original layout (excluding any additional outbuildings you have built since).

Other permitted development conditions are as follows:

  • The new extension cannot be higher than the tallest part of your roof, and no more than four metres as a maximum for a one-storey extension.
  • Extensions with two or more storeys cannot extend more than three metres past your original rear wall.
  • Single-storey extensions built on a side return cannot be wider than half of the original home.
  • A double-storey extension cannot be any more than seven metres from your boundary.

Note that exact rules can vary between local authority planning offices, so if your extension is close to the perimeter of your home, you may need to double-check that permitted development still applies.

Boundary Restrictions on Extensions Built Under Permitted Development

As listed above, an extension of more than one storey high cannot exceed the rear boundary of the building by more than three metres, although this doesn’t apply if another property is built on the land behind your home.

Most homeowners can build up to their side boundaries, depending on the neighbouring building and its proximity to the property.

Planning officers check whether an extension is lawful by using a ’45-degree rule’. They create an imaginary line between your home and your neighbours and check whether your elevation will intrude on the privacy or sight lines of the house next door.

The best option is to speak to your neighbours if you intend to build a side return extension up to your boundary and share the plans to ensure you address any concerns before the building work begins.

Generally, you can build up to 7.2 feet, or 2.2 metres without any issues.

Building an Extension With a Shared Boundary

If your home has a shared boundary with neighbours on both sides, you can usually:

  • Build an extension under permitted development or with full planning permission up to 50 mm from the boundary. This option often circumvents any party wall considerations.
  • Build up to the boundary with a party wall agreement.

The second scenario allows you the maximum space for your extension and can be beneficial if your neighbour is happy to sign an agreement, or wants a shared party wall where they can build their own extension.

In some cases, a neighbour may refuse to party wall consent and object to you building an extension up to your shared boundary.

Disputes can be costly and time-consuming, so avoiding any disagreement is preferable. If you need to take legal action, you will need to hire a party wall surveyor and pay for the costs for your neighbour (or both neighbours on either side).

The surveyor will draw up an agreement, although where a consensus still can’t be reached the main surveyor will need to act as an arbitrator.

Another option is shifting the extension back from the boundary far enough that the party wall requirements are irrelevant.


Designing an Extension Up to a Street Boundary

Homeowners that live at an end of terrace property have an additional consideration because their boundary is shared with the street rather than another private property.

You can usually build up to the boundary if this is flush with your existing external walls.

If other homes in your area have extensions built like this, it is very likely that you will be able to proceed with permitted development, provided your extension meets the other conditions.

Building Extensions in Restricted Development Areas

It can be more difficult to build an extension if you live in a listed building, conservation area, or green belt with separate rules and requirements for new building work or alterations to existing structures.

Conservation areas have stricter rules, although the exact requirements vary between regions.

Normally, an extension will need full planning permission and can be built up to neighbouring boundary lines, provided you have a party wall agreement.

If there is a shared boundary with a public road, permission is less likely, and the design will certainly need to match the character of the property and use local materials and style.

Greenbelt properties also need to meet limitations on the maximum size set by the local council. You will usually need to submit plans showing that the extension is set back from the boundary and cannot be seen by passersby or neighbouring properties.

There are exceptions to these rules, but you should check with the local council to confirm the precedents set in your area.

Other Factors When Building an Extension Close to the Boundary

Even if your extension complies with the permitted development rules or has full planning consent, it’s important to review your planned construction.

It is common to want to have as large an extension as possible, but you need to imagine how the layout will work and ensure you have sufficient space around the sides of your property to allow access to gardens and other areas.

You can also look at where the natural sunlight hits your home to avoid building an extension that will block the light from your ground floor living spaces or make your garden cooler and shadier than you would like.

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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