How close can you build your extension to your neighbours' boundary?

© Matt Gamble

_At a glance: _

- How close can you build to the boundary?
If you have Permitted Development Rights or Planning Permission, you can build as close as 50mm to your neighbour’s boundary.

- Do you need a Party Wall Agreement?
Yes, if you’re building on or near a shared boundary, a Party Wall Agreement is often required.

- What’s inside this guide?

• How close you can build to your neighbour’s boundary.
• Boundary Rules UK
• The importance of a Party Wall Agreement and how to handle it
• What to do next if you're planning your side extension.

When it comes to planning a side extension, it’s vital to establish whether you share a Party Wall with your neighbours. Before you start building, it’s crucial to understand the side extension boundary rules in the UK. Depending on the nature of your extension and the proximity of your existing property to the next door, it could have an impact on whether or not your proposed works can go ahead. In order to give your plans the best chance of going ahead, it’s important to know what constitutes a boundary, how it may influence your extension ideas and whether you need to secure a Party Wall Agreement.

Whether you're considering a small addition or a larger expansion, this guide will help ensure your project stays on track.

What is a boundary?

A boundary isn't always a physical structure like a wall or fence but refers to the point that separates one property from another. When planning a side extension for example, understanding the boundary line is crucial as it impacts how close you can build to your neighbour’s property.

How close to your neighbour’s boundary can you build your extension?

You can build your extension up to 50mm from your neighbours’ boundary as long as you have either Permitted Development Rights or have Planning Permission. This could be a suitable option if you aren’t able to secure a Party Wall Agreement.

Alternatively, if you are able to come to an understanding with your neighbour and secure a Party Wall Agreement, you’ll be able to build on the boundary line. This is our recommended approach if you’re hoping to extend your home to the maximum size possible.

Do you need a Party Wall Agreement?

What is a Party Wall Agreement?

A Party Wall Agreement is a contract between you and your neighbour that outlines an understanding regarding any building work that could impact a shared wall, boundary, or outbuilding. When it comes to side extension boundary rules, this agreement is vital if your extension might affect a boundary or shared structure.

When should you need one?

You may need a Party Wall agreement if you share a boundary wall with a neighbouring property or properties and the work is likely to affect them, ie if you're planning a side extension. If you’re lucky, you may receive written consent from your neighbour to say that they’re happy for the building work to go ahead. You should let them know two months ahead of the proposed start date. However, agreement is not guaranteed and it can prove to be a contentious issue.

What happens if your neighbour refuses your build?

Once you request compliance, your neighbours will have 14 days to respond to your request for whether they agree or disagree to it. Following this, they’ll be sent another letter requesting a response and allocated another 10 days to reply.

Failure to respond or refusal at this point could lead to you needing to have a surveyor to request a Party Wall Agreement. Your neighbour can request the opinion of a second professional to the one that you’ve selected for the task. As the responsibility for the legal bills falls is yours, you’re obliged to provide this for them.

In addition to cordially agreeing to cover their legal costs and securing them a second surveyor, our most significant advice is to be nice to your neighbours from the very beginning. The more transparent you are about your plans from day one, the more understanding your neighbours are likely to be.

Learn more about the next steps for how to serve a Party Wall Notice.