Section 6 asks two questions at once: how close your excavation is to the adjoining owner's structure (the neighbour's wall, floor or ceiling with an interest in the shared boundary), and how deep it goes relative to that structure's foundations. Being close on its own proves nothing. A notice under section 6 only arises once both conditions in the same test are met.

The three-metre test, section 6(1)

Section 6(1) sets two conditions, and both must be satisfied. The excavation has to be within three metres, measured horizontally, of the adjoining owner's building or structure. It also has to go deeper than the bottom of that structure's foundations.

Meet the distance alone, without the depth, and section 6(1) does not apply. Meet the depth alone, without the distance, and it does not apply either.

The six-metre test, section 6(2)

Section 6(2) covers ground a little further out. It applies where the excavation is within six metres, again measured horizontally, and where a line drawn downwards from the neighbour's foundations at forty-five degrees would meet your excavation.

This is a plane, so a deep excavation can trigger it at a horizontal distance that would look safe on a tape measure alone.

Whichever test applies to your job, both its conditions need checking together. Neither section works from distance by itself.

Section 6(1) sets two conditions and both have to be true at once. The excavation has to sit within three metres of the neighbouring building, measured horizontally, and it has to go deeper than the bottom of that building's foundations. A trench a long way off but very deep does not trigger the notice on its own, and nor does a shallow one right up against the boundary. Only the combination does.

Working out the three-metre test on your own job

Start with the horizontal distance. Measure from the edge of your proposed excavation to the nearest point of the adjoining owner's structure, the neighbour's building or wall that has an interest in the works. If that distance is more than three metres, section 6(1) does not apply and you can stop there.

If it is three metres or less, check the depth. Find out how deep the neighbouring structure's foundations go, and compare that to how deep your excavation will reach. Section 6(1) only applies where your excavation goes below the level of their foundations. Above that level, even close in, this limb is not met.

Working out the six-metre test

Section 6(2) covers a wider zone with a different shape. It applies where the excavation is within six metres of the neighbouring structure, and where a line drawn downwards from the bottom of that structure's foundations at forty-five degrees would be cut by your excavation. Picture the forty-five-degree line running down and outward from the base of the neighbour's foundations: if your dig crosses that line, section 6(2) applies even though you are further away than the three-metre test allows.

The two tests are checked separately. An excavation can fail the three-metre test and still be caught by the six-metre plane, so it is worth working through both before deciding a notice is not needed.

Section 6(3) of the Party Wall etc. Act 1996 lets the adjoining owner (the neighbour whose building stands near your excavation) require you, the building owner (the person carrying out the work), to underpin, thicken or otherwise strengthen the foundations of their building where your dig makes that necessary. This applies once either of the section 6(1) or section 6(2) tests is met, so it can arrive as soon as your notice does.

What the duty actually adds to your job

Underpinning is extra construction, not a formality. It has to be designed, sequenced and carried out before or alongside your own excavation, and it has to happen even if you had not planned for it when you priced the job.

Who pays, and what it does to your timeline

Section 11(1) makes the building owner liable for the cost of works carried out under the Act, so underpinning required under section 6(3) is paid for by you. Where the two of you cannot agree the detail of how it is done, that detail is settled in an award, the document produced under section 10 when a dispute is resolved. Deciding whether you need a surveyor to reach that award is worth doing early, and the page on that question sets out how the appointment works.