Three questions to ask about your extension
1. Does it touch an existing shared wall? If the extension cuts into the party wall, bears steelwork on it, or raises it, that is party structure work.
2. Is any new wall being built on or up to the boundary? A new flank wall on the boundary line needs a line of junction notice.
3. How deep are the foundations, and how close to next door? Foundations closer than 3 metres to next door and deeper than theirs, or closer than 6 metres and deep enough to cross the 45-degree line, need an excavation notice. See the 3 and 6 metre rules.
Answers for typical extensions
| Extension | Likely notices |
|---|---|
| Single-storey rear extension built on the boundary | Line of junction, excavation, often party structure where it ties into the house |
| Rear extension 1 metre in from the boundary | Excavation |
| Side return extension in a terrace | Line of junction and excavation |
| Two-storey side extension of a detached house, 4 metres from neighbour | Usually none, unless foundations are deep |
| Wraparound extension | Often all three |
When planning permission is not needed
Many extensions are permitted development and need no planning application. That has no bearing on the Party Wall Act, which applies regardless.
Before you commit to a start date
Ask your architect or designer to mark the foundation depth and position on the drawings. That single detail decides most excavation questions. Then build the notice period into your builder's programme.
Where this comes from in the Act
- Section 1: New building on line of junction
- Section 2: Repair etc. of party wall: rights of owner
- Section 6: Adjacent excavation and construction
Links go to the official text of the Party Wall etc. Act 1996 on legislation.gov.uk.