How Close to My Boundary Can I Build an Extension?

How Close to My Boundary Can I Build an Extension?

There are a number of reasons to consider an extension if you’re looking to increase value, space or even add a bit of light.

If you’re interested in extending a property, it’s crucial that legal requirements are considered.

These legalities can include what type of property is in question, the surroundings and not to mention where the property is based. These are important factors as they play an essential role in whether the property can be extended and how far out you can build.

If you find yourself in need of expert advice, you can contact the Cornwall structural designers at Martin Perry Associates; we’d be happy to respond to any concerns and queries you may have.

In the meantime, we’ve decided to focus on a few factors to clear up any confusion. Here, we’ll be looking into how close you can build an extension to the property boundary.

An aerial view of a suburban area

What is a Boundary?

To start with, there are two types of boundaries. In English and Welsh property law, there are legal boundaries and physical boundaries.

The legal boundary is a line on the property plan or description of the area that divides one property or land from another.

The physical is much easier to determine as it is an object or barrier that separates the property from another. This can be a fence, wall or hedge.

Considerations Before Building an Extension

Although the space of an extension is always desirable, it’s worth noting a few factors before you start knocking down any walls.

Site Access

It sounds obvious but it’s easily overlooked. When the construction starts to take place, lorries and machinery will likely need to gain access to your site.

You’ll need to consider how they will be able to do this: where will they park, and where will they store their tools? This forward thinking will help avoid any inconvenient bumps along the road.

A line of terrace houses

Shared Walls

You might be dreaming of your extension but you’re holding back because of the shared walls. If the property is a terrace or semi-detached, you are likely to find that you will need to comply with the Party Wall Act.

The construction work will need to comply with the Party Wall Act if:

  • The work involves excavation within 3m of a party wall/party fence to a level below its foundations.
  • The work consists of digging foundations within 6m of a party structure to a level below a 45° line from the base of its foundations.

The Act is in place to ensure any work undertaken by a neighbour doesn’t affect the structural integrity of the shared walls or neighbouring properties. It also aids in avoiding any future disputes between neighbours, if any should arise.

If any disputes do occur, the Act should help to effectively resolve them. You can find a helpful guide on the Party Wall Act on our website.

If you find yourself in need of a party wall agreement, then we recommend having a chat with your neighbours about it before you start posting any party wall agreements through your neighbour’s letterbox.

The Demand on Your Utilities

The electrics, plumbing, heating and gas may cope well with supplying enough utilities to meet the demands of the running property, but is it sufficient for an added extension?

To avoid this being a problem, you’ll want to give the current services a health check to ensure the extra demand will be met with ease.

An architect drawing some plans

Miscellaneous Factors

These may sound like small or inconvenient thoughts to think about, but they are just as vital when considering the possibility of an extension. These include:

  • Ground conditions
  • Obstructive trees
  • A history of flooding

Ground conditions are important to ensure the structure is securely supported and the foundations are suitably designed.

Tree’s might sound a bit silly in comparison to other factors, but they can be an obstruction which causes issues down the line.

It’s never a bad idea to backtrack and read up on the history of the property, especially flooding. It’s best to be in the know and prepared for these factors as they can cause issues when building on the property.

A fence

How Close to the Boundary Can a Property be Extended?

The rules and regulations aren’t always user friendly in terms of understanding this information.

You cannot breach the boundary in the plan in elevation. This includes the 45° rule that most planning officers abide by.

They will use the closest window to your house from your neighbours’ property to measure. If this breaches the legal boundary line, then planning permission can be rejected.

If there is no other house or estate on the land behind your property and you plan on building an extension that is more than one story, you will be restricted to 3m beyond the rear boundary.

Single story extensions that are built to the side of your property will have a maximum height of four metres and be no wider than half the width of the property.

If you are building a double extension, you will not be able to go any closer than 7m to the boundary line.

We hope you now have a better understanding of how close you can build to your boundary. For more information on extending your property, take a look at our blog that looks at how far you can build without permission.

Here at Martin Perry Associates, we offer structural designs for a range of property work such as loft conversions and extensions. If you are looking for support, please don’t hesitate to contact us. You can call on 01579 345777 or email us at

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