Section 6 is the part of the Act people are most surprised by, because it applies where there is no shared wall at all. If you are digging near the building next door and going deeper than its foundations, the Act wants the neighbour told, whether or not you ever touch their property. Two tests decide it, and they are geometric rather than a matter of judgement, which is unusual and helpful. Both are measured horizontally from any part of the neighbour's building or structure, not from the boundary.
The three metre test
The first test asks whether you are excavating within three metres, measured horizontally, of any part of the adjoining owner's building or structure, and whether any part of the excavation will go to a lower level than the bottom of that building's foundations. If both are true, notice is owed. This is the one commonly called the three metre rule, and it catches most rear extensions with trench fill footings, because a modern footing goes deeper than a Victorian one.
The six metre test and its forty-five degree line
The second test extends to six metres, but only where the excavation cuts a plane drawn downwards at forty-five degrees to the horizontal from the bottom of the neighbour's foundations at the face of their external wall. In plain terms: the further away you dig, the deeper you have to go before the neighbour's ground is affected. This is the test that catches basements and deep piles, and it is the one where a drawn section, rather than a tape measure, settles the question.
What the notice has to say, and what follows
A section 6 notice goes out at least one month before you begin to excavate, and it has to state your proposals and whether you intend to underpin or otherwise strengthen the neighbour's foundations. Plans and sections showing the site and the depth accompany it. The adjoining owner may require you to underpin at your expense so far as necessary, which is the practical reason to know early whether your footings pass the tests, and to design accordingly rather than discovering it on site.
Questions people ask about party wall act section 6
Does the three metre rule measure from the boundary?
No, and this is the most common error. It measures horizontally from any part of the neighbour's building or structure. A house set well back from the boundary changes the answer completely, in both directions.
What counts as a structure?
More than a house. An outbuilding, a substantial garden structure or a retaining wall can be a structure for these purposes. If it has foundations and is next to your dig, it is worth measuring to.
Do I need a notice if my footings are shallower than theirs?
Not under section 6, because the depth limb of both tests fails. Get the levels checked rather than assumed: a trench fill footing beside a shallow Victorian one usually goes deeper than people expect.