The Party Wall Act deals with wall matters and excavations done by the building owner in proximity to the adjacent owner's property.
When an owner intends to excavate within 3 meters of the adjoining property's wall/foundation and intends to go deeper than the depth of the neighbour’s foundation, then a section 6 notice must be served.
Section 6 notices are only valid when they contain certain specific information. One such requirement is a section drawing, which illustrates what is intended in the way of foundation digging. Namely, the drawings have to show the new foundation at a depth that would be lower than the adjoining owner's foundation and how close the new excavation would be to the neighbouring property and the type of foundation used.
A good assumption about how deep a wall/foundation the adjoining owner has can generally be made by an experienced party wall surveyor.