The short answer
Usually, yes. A single-storey rear extension most often triggers the Act through its foundations: digging for footings within three metres of the neighbour’s building and deeper than their foundations brings in section 6. If the extension also ties into, is built off, or cuts into the party wall, section 2 applies too. You will need to serve the right notice or notices before starting — even with planning permission, and even on a detached house where the excavation rule is met.
Why it matters
Most rear extensions run along the boundary with one or both neighbours, so the new foundations sit close to their wall — a section 6 notice of adjacent excavation (one month). If you build the new flank wall up to the boundary, that is section 1 (line of junction). If you cut into the existing party wall for the new roof, flashings, or to bond the new wall, that is a section 2 work, requiring a party structure notice under section 3 (two months). On a terrace you may have two adjoining owners. A detached house is not exempt from section 6 if the excavation is close and deep enough.
What to do now
- You can both appoint one agreed surveyor instead of one each — the impartial, lower-cost route Coburns recommends.
- Get your foundation drawings, with the proposed depth.
- Check the distance from the excavation to each neighbour’s wall.
- Apply the section 6 tests, and check whether you are building on the boundary or cutting into the party wall.
- Serve the right notices, with the correct periods, before you start.
Common mistakes
- Assuming a single-storey 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 a single-storey rear extension needs, and serve them free of charge.