The short answer
Usually yes, but typically on fewer counts than a taller extension. A single-storey extension most commonly triggers the Act through its foundations: digging footings within three metres of a neighbour’s building and deeper than their foundations needs a notice of adjacent excavation (section 6). If it ties into or cuts into the party wall, a party structure notice (section 3) applies; and if a new wall is built on the boundary, that is a line of junction matter (section 1). Because there is no upper floor bearing into the party wall, single-storey work is often lighter on party structure issues than a double-storey extension.
Why it matters
For a single-storey extension the foundations are usually the key trigger:
- Foundations (section 6) — the most common one, with one month’s notice.
- Party wall works (section 2; party structure notice under section 3) — where you cut in, bond to or flash into the party wall. Two months’ notice.
- Building on the boundary (section 1) — a new wall on the line. One month’s notice.
The main point for your programme is the two-month party structure period, if any party wall work is involved — so serve early. A double-storey version of the same extension adds the upper-floor party wall work on top.
What to do now
- Get the foundation depths and apply the section 6 tests.
- Check for any party wall ties (section 2/section 3) and any boundary wall (section 1), and serve the right notices.
- Allow for the two-month period if there is party wall work.
- 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.
Common mistakes
- Assuming a single-storey extension is too small to notify.
- Checking only the three-metre test.
- Forgetting flashing cuts into the party wall.
- Missing the second neighbour on a terrace.
When to call Coburns
Send us your plans and we will confirm exactly which notices your single-storey extension needs, and serve them free of charge.