Extensions and party walls
Rear and side extensions often need party wall notices: for building a wall at the boundary, for digging foundations near the neighbour's house, or both. Here's how to tell, and what to expect.
Building on or up to the boundary (section 1)
If your extension wall will be built on the line of the boundary, you must serve a section 1 notice at least one month before starting. There are two options:
- Wholly on your land, right up to the boundary. Your neighbour can't prevent it, but notice is still required. Where necessary, the foundations may project under your neighbour's land, at your expense, and you must make good any damage this causes.
- Astride the boundary, as a new party wall. This is only possible with your neighbour's written agreement. If they agree, the cost is usually shared in proportion to how each of you uses the wall.
Reinforced concrete "special foundations" that extend under the neighbour's land need their written consent.
Digging foundations near the neighbour (section 6)
A section 6 notice is needed, at least one month before starting, if you'll excavate:
- within 3 metres of a neighbouring building or structure, to a depth below the bottom of its foundations, or
- within 6 metres, if the dig cuts below a 45° line drawn down from the bottom of the neighbour's foundations. This usually applies only to deep foundations or basements.
Section 6 often applies to older properties. Victorian, Edwardian and interwar houses usually stand on shallow, stepped brick footings, often only 450–600 mm deep. A modern extension foundation is typically around a metre deep or more. So building within 3 metres of an older neighbouring house nearly always means digging below its foundations, and a section 6 notice is needed. See what foundations to expect by house age →
The notice must include plans and sections showing the excavation. Without them, the notice may be invalid.
Work to the existing party wall (section 2)
Many extensions also involve the existing party wall or the neighbour's extension wall. For example: cutting in a flashing, bearing a new beam on the wall, or raising a shared garden wall. That needs a section 2 notice, with two months' notice.
Gutters and eaves. The Act doesn't let any part of your extension, such as gutters, fascias or roof overhangs, project over the boundary without your neighbour's agreement. Design the eaves to stay on your side.
Typical combinations
| Project | Likely notices |
|---|---|
| Single-storey rear extension built against the neighbour's extension | Section 2 (flashing or work to their wall) and section 6 (foundations) |
| Rear extension with its wall on the boundary | Section 1 and section 6 |
| Side extension to a semi, up to the boundary | Section 1 and section 6 |
| Extension set well away from the boundary, more than 3 m from the neighbour's building | Often none. Check the foundation depth and distance. |
Costs
Notices are £100 per neighbour, covering all the notices they need. If a neighbour dissents, an award starts from £995, plus a schedule of condition from £390. The building owner normally pays the surveyors' fees. See full pricing.
Extension questions
Do I need a party wall notice for a single-storey extension?
Often, yes. The size of the extension doesn't decide it: what matters is whether you build at the boundary, dig near the neighbour's foundations, or work on a shared wall.
My neighbour's garage is next to the boundary. Does section 6 apply?
It can. Section 6 applies to a neighbouring "building or structure", not just their house. A garage or outbuilding within 3 metres of your dig, with shallower foundations, can trigger it.
Can I build my extension wall on the boundary without my neighbour's permission?
You can build wholly on your own land, right up to the boundary, after serving a section 1 notice. You can't build astride the boundary without your neighbour's written agreement.
Not sure if the Act applies to your project?
Tell us what you're planning. We'll tell you, free of charge, whether you need to serve notice and what happens next.