The short answer
Often, yes — but it depends on the work, not on the word “extension”. You need a party wall agreement (properly, an award) only where your extension involves notifiable work under the Party Wall etc. Act 1996: cutting into or building off a shared (party) wall, building a new wall on the boundary, or excavating close to your neighbour’s building. Planning permission and permitted development make no difference, and even a detached house can be caught by the excavation rules.
Why it matters
There are three triggers to check:
- Work to the party wall itself (section 2) — inserting a beam, raising, thickening or rebuilding the wall, or removing a chimney breast on the party wall. This needs a party structure notice, served two months before work starts.
- Building on the boundary (section 1) — a new wall up to or astride the line of junction needs a line of junction notice, served one month before.
- Excavation near your neighbour (section 6) — digging within three metres of their building and deeper than their foundations, or within six metres where a line drawn down at 45° from the base of their foundations would be cut. This needs a notice served one month before.
Most extensions touch at least one of these. A rear extension usually triggers the excavation rules and sometimes the party wall rules; a side return often runs new foundations right alongside the boundary; a two-storey extension frequently involves raising the party wall; and a basement almost always engages the Act. If any trigger applies, you must serve the right notice before you start — getting it wrong can leave you without the Act’s protection and exposed to an injunction.
What to do now
- If a surveyor is needed, use one. Both owners can appoint a single impartial ‘agreed surveyor’ rather than one each — quicker, cheaper and less adversarial. Coburns recommends a single agreed surveyor wherever possible.
- List exactly what the works involve and check them against the three triggers above.
- Identify every adjoining owner, including freeholders and any flats above or below — not just the person next door.
- Serve the correct notice (or notices) with the full notice period before your planned start date.
- If you are not sure whether the Act applies, send your drawings to a surveyor before you commit to a start date.
Common mistakes
- Assuming planning permission or permitted development removes the duty — the Act is entirely separate.
- Assuming a detached house is exempt — the excavation rules can still apply.
- Describing the work too loosely, so the notice is defective.
- Starting work before the notice period has expired or the dispute is resolved.
- Forgetting to serve a relevant owner, such as the flat below or a separate freeholder.
When to call Coburns
Send us your plans and we will tell you, free of charge, whether the Act applies and exactly which notices you need — then prepare and serve them for you. Getting this right at the start is the cheapest way to keep your project on track.