Extensions: maximising space with eccentric foundations

Eccentric Foundations – a guide

If you live in a semi-detached or terraced property you have a right to build up to the boundary, and ‘if necessary’, a right to put foundations on your neighbour’s land, however, you will almost certainly need party wall agreement. So, for example, if you want to extend to the back or side of your house you must get a party wall agreement. Your neighbours can say no to you putting foundations on their land and in that case, you will need to change the design of the foundation.

There is a type of foundation, an eccentric, or offset foundation, that allows you to build on the boundary line without worrying about disturbing your neighbour’s land. Even so, you must ensure you get all party wall agreements in place beforehand and ensure you have the relevant planning permission.

What are eccentric foundations?

In nearly all extension builds, an eccentric foundation can be used, preventing you from going into your neighbour’s property. An eccentric foundation is where the wall sits on the outer edge of the foundation. This means you can dig the trench for your foundations right next to the boundary and not over it because the wall of your extension will sit on the edge of it.

How is it different to traditional foundations?

With traditional, or typical foundations, the wall sits on the centre of the foundation.

Eccentric Foundations

When are eccentric foundations used?

There are four key areas when an eccentric foundation will be the preferred option:

  1. When you want build against the boundary line (the line of junction in the Party Wall Agreement) and your neighbour refuses permission to put foundations on their land.
  2. If your neighbour already has an extension meaning you can’t possibly have a traditional foundation.
  3. If your neighbour’s foundation already falls into your land meaning a traditional foundation cannot be built. 
  4. Where your neighbour has responded to your party wall notice and stated they do not want anything built within their land because they may themselves want an extension in the future.

Do I still need party wall agreement from my neighbour if my extension has eccentric foundations?

Yes. The party wall agreement is not just for if your planned extension touches the boundary line. We have written extensive articles on the Party Wall Act etc this one gives a broad overview.

Are eccentric foundations mandatory for party wall projects?

No, they are not mandatory. You can still use traditional foundations as long as your neighbour has agreed and they can be built without restriction (see ‘When are eccentric foundations used, points 2 and 3. 

If you want more advice about party wall agreements for an extension, or any other reason, please get in touch.

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