Executive Summary:
People often treat the Act’s excavation distances as if they were engineering approval. They are not.
This article explains what the 3 metre and 6 metre triggers actually do, what the 45 degree line means in practice, and why good design and method planning still matter.
What the excavation sections are really doing:
The Act creates a notification trigger when excavation is close enough to potentially affect a neighbouring building or structure. One part applies within 3 metres with no depth limit, and another part covers excavation within 6 metres where the excavation would intersect a 45 degree plane taken from the bottom of the neighbour’s foundations. Those triggers tell you when to notify, not how to excavate safely.
Why the 45 degree idea can mislead:
The Act’s wording can look like it assumes you can excavate down to foundation level within 3 metres and then continue outside a 45 degree line. The text itself notes that the engineering justification is not obvious and may relate loosely to how soils behave when they form a natural slope. That is a warning sign, not reassurance.
Soils do not behave like a neat diagram:
Loose granular materials form a slope based on friction and cohesion. That is sometimes called an angle of repose, but a slope at the failure point has no spare margin. Real excavations disturb ground conditions, loads change, water shows up, and what looked stable can move quickly. The point is not to fear excavation, but to avoid assuming the legislation diagram equals a safe excavation method.
What a sensible approach looks like:
Treat the Act as the process trigger, then treat engineering as the safety answer. If the work is notifiable, provide proper plans and sections with the notice, and make sure the design addresses the actual site conditions. If your neighbour is worried, good information and a credible method reduce the chance of dissent and delay.
DIG DEEP, PLAN DEEPER:
If your project involves excavation near a neighbour, do not gamble on rules of thumb. Lead with proper drawings, clear method planning, and professional advice so the conversation stays calm and the ground stays put.