The short answer
Most rear extensions are notifiable. The common trigger is the 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 the extension ties into or cuts into the party wall, that is party structure work — a party structure notice under section 3. And if a new flank wall is built up to the boundary, that is a line of junction matter (section 1). This holds whether the extension is single or double storey, and even on a detached house for the excavation point.
Why it matters
Rear extensions usually run along the boundary with one or both neighbours, so the triggers stack up:
- Foundations (section 6) — the usual one, with one month’s notice.
- Party wall works (section 2; party structure notice under section 3) — where you cut in or bear beams, bond the new wall, or cut in flashings. Two months’ notice.
- Building on the boundary (section 1) — where the new wall sits on or astride the line. One month’s notice.
A double-storey rear extension adds more party wall work, because the upper floor is usually tied into the wall. On a terrace you may have two adjoining owners, and a detached house is not exempt from section 6 if the excavation is close and deep enough.
What to do now
- Get your foundation drawings, with the proposed depth.
- Check the distance from the excavation to each neighbour’s wall, and apply the section 6 tests.
- Identify any party wall ties (section 2/section 3) and any new wall on the boundary (section 1), and serve the right notices with the correct periods.
- 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 rear extension is too small to be notifiable.
- Checking only the three-metre test and missing the six-metre / 45° one.
- 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 rear extension needs, and serve them free of charge.