When an extension triggers the Party Wall Act
Three notice types apply to extensions. A Party Structure Notice is required if you are inserting a steel beam into a party wall (almost every rear extension that opens up the existing rear wall to the new extension), cutting into a party wall for joists or padstones, or raising/underpinning a shared wall. A Line of Junction Notice is required if you are building a new wall up to or on the boundary with your neighbour — common for side returns where the new wall sits on the existing boundary. A Three Metre Notice is required when you excavate foundations within 3m of a neighbour's structure, measured horizontally — almost universally triggered for London extensions because terraced properties are closely spaced. A Six Metre Notice applies where foundations are deeper than the neighbour's by more than the horizontal distance — less common.
The party wall process for extensions
Step 1: Identify which notices are required (your contractor or surveyor advises). Step 2: Serve the notice on each adjoining owner at least 2 months before structural work starts (1 month for excavation-only notices). Step 3: 14-day response window. The neighbour can consent in writing (no surveyor needed, but a schedule of condition is still advisable), dissent (triggers surveyor process), or fail to respond (deemed dispute, triggers surveyor process). Step 4: If dispute, each owner appoints a surveyor (or agrees a single 'agreed surveyor'); the surveyors then prepare a Party Wall Award recording pre-construction condition, agreed working method, hours, protection measures and dispute resolution. Step 5: Work proceeds in accordance with the Award. Step 6: On completion, surveyors confirm no damage; if damage occurred, the Award provides for repair or compensation.
Costs and timelines
Party wall surveyor fees on a typical London extension: agreed surveyor route £700–£1,500; separate surveyors £1,200–£2,500 total (one per party plus potential third surveyor). The building owner (person doing the works) pays all surveyor fees. Timeline: notice served at design completion (typically 6–10 weeks before construction start). 14-day response. If agreement needed, surveyors appointed within 10 working days; Award typically issued 3–6 weeks after appointment. Total party wall process: 4–10 weeks from notice to Award. Builderr serves notices as part of standard project management; surveyor fees are additional and quoted separately.
