When does the Party Wall Act apply?
The Act applies in three scenarios. Section 1 — building on or astride a boundary (a new wall on the line of junction with the neighbour's land). Section 2 — works to an existing party wall, including cutting in for steel beams, raising height for a loft mansard, demolishing and rebuilding, or underpinning. Section 6 — excavations within 3 metres of a neighbour's building where the dig goes deeper than the neighbour's foundations, or within 6 metres where a 45-degree line from the bottom of the deeper foundation would intersect the new excavation. Almost every London side return, rear extension, basement and loft conversion triggers at least one of these. Failing to serve notice does not invalidate the works but exposes you to civil injunction stopping the build, plus liability for any damage with reversed burden of proof. Builderr serves all Party Wall notices on behalf of the client before site start.
Notice periods and the dissent route
Section 1 notices need 1 month before start. Section 2 and Section 6 notices need 2 months. The neighbour has 14 days to respond with three options: consent in writing (no surveyor needed, you proceed), dissent and appoint their own surveyor, or fail to respond (deemed dissent — you then write again and if they still don't appoint, you appoint a surveyor on their behalf). Once dissent triggers, the two surveyors (or an agreed single surveyor) inspect both properties, agree the works specification, photograph existing conditions to record any defects (the Schedule of Condition), and issue a Party Wall Award setting out method statement, working hours, access arrangements and a damage settlement procedure. The Award is legally binding on both parties.
Common London scenarios
Side return extensions trigger Section 1 (new wall on the boundary) and often Section 6 (excavation within 3m of the neighbour's house). Rear extensions trigger Section 6 wherever foundations go below the neighbour's footing depth. Basement digs trigger Section 6 across multiple neighbours and require very detailed engineering submissions in the Award. Loft mansards and hip-to-gable conversions trigger Section 2 if a steel beam bears into the party wall. Two-storey side extensions trigger Section 1 (new boundary wall) and frequently Section 2 (raising the existing party wall height). Internal works that don't touch the shared wall — like a Velux loft conversion bearing only into the host roof — typically do not trigger the Act. The Act applies to terraces, semis, flats and listed properties identically.
