If your neighbour is proposing a rear extension, whether they must serve you (the Adjoining Owner) with a Party Wall Notice depends on distance, depth, and what works are planned. Below is a practical guide to when the Party Wall etc. Act 1996 applies in England & Wales.
1) Distance, Depth & the 45° test
- Beyond 6 metres: Party Wall Notice is not usually required.
- More than 3 metres but less than 6 metres: A notice may be required if the proposed excavation triggers the 45° rule.
- Within 3 metres: A notice is often required if the new foundations will go deeper than yours.
The 45° rule (Section 6):
Imagine a line at 45° from the bottom of the neighbour’s new excavation towards your foundations. If that line intersects the base of your foundations, the work is notifiable.
Don’t know your foundation depth? For older homes (e.g., late-Victorian/Edwardian/1930s), typical assumptions are 300–600 mm, but surveyor input is sensible.
2) Which parts of an extension are “notifiable”?
Three sections of the Act can be engaged by a rear extension:
Section 1 – New wall at/over the boundary (line of junction)
- Covers building up to or astride the boundary.
- Commonly engaged by flank walls of an extension close to the boundary.
- Notice period: At least 1 month before works begin.
Section 2 – Works to existing party structures
- Applies to works on a party wall, party fence wall (shared garden wall), or party structure (e.g., floors/ceilings in flats).
- Includes: underpinning, cutting in for steel beams, removing projections (e.g., chimney breasts), rebuilding, damp-proof courses, raising, etc.
- Less typical for standard rear extensions unless you are altering a shared wall.
- Notice period: At least 2 months before works begin.
Section 6 – Excavations near your structure
- Applies to excavations within 3 metres (and deeper than your foundations), or within 6 metres where the 45° plane to your foundations is crossed (e.g., piled/deep foundations).
- Notice period: At least 1 month before works begin.
Note: Modern extensions commonly have ~1 m deep foundations (often deeper near trees/granular soils/subsidence areas), so Section 6 is frequently engaged where homes are close together.
3) What if only part of the scheme is notifiable?
Only the elements that fall under Sections 1, 2, or 6 require notice. Other non-notifiable works can proceed without a party wall notice—but most real-world extensions involve excavation that triggers Section 6.
4) Timings & next steps for Adjoining Owners
- On receipt of a notice, you usually have 14 days to respond (consent, dissent with an Agreed Surveyor, or dissent with your own surveyor).
- If you don’t respond, the Building Owner can appoint a surveyor for you under Section 10(4) to keep the process moving.
- A Schedule of Condition and a Party Wall Award (agreement) will typically follow where there’s a dissent.
Quick checklist for Adjoining Owners
- How close is the proposed extension to my property (3 m / 6 m rule)?
- How deep are my foundations likely to be (older homes often shallow)?
- Are they building at/over the boundary (Section 1)?
- Are they cutting into/altering any shared walls (Section 2)?
- Will their excavation intersect the 45° line to my foundations (Section 6)?
- Have I received a valid written notice with the correct minimum notice period?
Want a fast, low-cost view on whether a notice is required—and what to do next?
Email team@simplesurvey.co.uk. Simple Survey are the lowest-cost party wall surveyors across England & Wales. We’ll confirm if your neighbour’s extension is notifiable, review any notice you’ve received, and set out your options clearly and affordably.