Does s6 apply to garden walls + outbuildings?

Yes — s6 refers to any 'building or structure' of the adjoining owner. Neighbour's garden wall, garage, outbuilding all count. A new extension foundation 2m from neighbour's garden wall, excavating below the wall's footing, triggers s6. Often overlooked at design stage — flagged late by surveyors mid-build. Build into design check from outset.

What if I'm excavating piled foundations not strip foundations?

s6 applies to ANY excavation including pile bore holes. CFA piles + mini-piles within 3m or 6m of neighbour structure trigger s6 if depth criteria met. Often more conservative engineering required — pile shafts within the 45° zone may require sleeving, displacement piling vs CFA bore, or stepped foundation design. Party wall surveyor + structural engineer coordinate design + Award methodology.

Does the 3m or 6m measure from the building or the boundary?

Always from the adjoining owner's BUILDING or STRUCTURE, not the boundary line. A boundary 1m from neighbour wall: the 3m s6 jurisdiction extends 3m from the wall = 2m on the building owner's side beyond the boundary. Where adjoining building sits far back from boundary, s6 trigger may not apply even for excavation close to boundary. Always measure from the structure.

Party Wall 3m vs 6m Section 6 London | Builderr

What's the Difference Between the 3m and 6m Rules for Section 6 Notices?

Section 6 of the Party Wall Act 1996 has two distinct triggers. The 3m rule: excavation within 3m horizontally of any part of a neighbour's building or structure, to a depth equal to or below the neighbour's foundation underside. The 6m rule: excavation within 6m horizontally of the neighbour's building, to a depth below a notional 45° line drawn downward from the underside of the neighbour's foundations. Either trigger = 1-month s6 notice required. Common in basements + deep extension foundations.

The 3m rule — vertical alignment

The 3m rule (s6(1) Act) catches excavation within a horizontal 3m radius of any part of a neighbour's building or structure, where the excavation depth at any point reaches or exceeds the level of the underside of the neighbour's foundations. Example: typical Victorian London terrace where neighbour foundations are at -1.0m below ground; a side return extension foundation 2.5m from the neighbour wall excavating to -1.2m triggers s6 3m rule (within 3m + deeper than neighbour foundation). A house extension foundation at 1.5m horizontal distance excavating to -0.9m does NOT trigger (within 3m but shallower than neighbour foundation). Common triggers in London residential: most rear extension strip foundations excavating to -1.0m to -1.5m in proximity to neighbour walls, garage conversions with new strip foundation deeper than existing, basement preparation work. Test at design stage: section through proposed foundation + neighbour foundation; measure horizontal + vertical relationships.

The 6m rule — the 45° downward line

The 6m rule (s6(2) Act) extends jurisdiction further but only for deeper excavations. Catches excavation within 6m horizontally of the neighbour's building, where the excavation reaches a depth BELOW a notional 45° line projected downward + outward from the underside of the neighbour's foundations. Example: neighbour foundation underside at -1.0m; 45° line drops outward — at 2m horizontal distance the line is at -3.0m; at 4m horizontal it's at -5.0m; at 6m horizontal it's at -7.0m (with 6m being the outer limit of jurisdiction regardless of depth). Visualise as a 45° invisible wedge dropping outward from neighbour's foundation underside — anything excavated below this wedge within 6m triggers notice. Almost universally relevant to basement construction; rarely triggered by standard extension foundations.

Practical application + design discipline

Builderr design checklist at RIBA Stage 3: (1) plan view — measure horizontal distance from every proposed excavation edge to every adjoining structure (boundary walls, neighbour houses, neighbour outbuildings, neighbour basement light wells); (2) section view — measure proposed excavation depth + draw 45° lines from neighbour foundation undersides where excavation falls within 6m; (3) where multiple neighbours, run check for each separately — corner sites + mid-terrace can have 3–6 adjoining structures triggering s6 separately; (4) flag any trigger to client + party wall surveyor at planning stage; (5) on basements, ALWAYS expect s6 triggered for all adjoining properties — design programme + budget around this. Cost implications: s6 notice itself free/£185 to draft; if dissented, Award process £1,500–£4,500 + VAT/side standard, £3,500–£12,000 + VAT/side basement. Engineering: trigger of s6 often drives foundation design (mini-piles, contiguous piled wall, raft alternative) — choose foundation type that minimises adjacency risk + s6 cost burden. London Clay specifics: foundation depth typically -1.0m to -1.8m due to seasonal shrinkage; s6 3m rule almost always triggered for adjoining party walls on rear extensions. Plan for it.