Foundations are often what brings an extension within the Act
Excavation within 3 metres can be covered where the new work goes deeper than the neighbour's foundations. A separate 6-metre test can also apply depending on depth and geometry.
Boundary walls are another trigger
Building a new wall on or at the boundary can require notice. Work to an existing party wall, including cutting in structural bearings, can also be covered.
Silence is not consent
The government guidance states that neighbours must respond to notice and that no reply does not simply mean agreement.
The Act does not grant planning permission
Party wall procedures deal with rights and disputes between owners. Planning permission and Building Regulations remain separate.