The party wall 3 metre and 6 metre rules

The two tests, in the Act's words

This section applies where (a) a building owner proposes to excavate, or excavate for and erect a building or structure, within a distance of three metres measured horizontally from any part of a building or structure of an adjoining owner; and (b) any part of the proposed excavation, building or structure will within those three metres extend to a lower level than the level of the bottom of the foundations of the building or structure of the adjoining owner.

This section also applies where (a) a building owner proposes to excavate, or excavate for and erect a building or structure, within a distance of six metres measured horizontally from any part of a building or structure of an adjoining owner; and (b) any part of the proposed excavation, building or structure will within those six metres meet a plane drawn downwards in the direction of the excavation, building or structure of the building owner at an angle of forty-five degrees to the horizontal from the line formed by the intersection of the plane of the level of the bottom of the foundations of the building or structure of the adjoining owner with the plane of the external face of the external wall of the building or structure of the adjoining owner.

Reading the tests

Distances run from their building, not from the boundary. Both tests measure horizontally from any part of the neighbour's building or structure. A building can count even if it is not next door: under section 6(4), owners of buildings within these distances are treated as adjoining owners for this section, and the booklet notes this can include a building with another owner's land in between.

The 3 metre test compares depths. Within 3 metres, the only question is whether any part of your excavation, foundation or structure will be lower than the bottom of their foundations.

The 6 metre test uses a sloping line. Take the level of the bottom of their foundations and find where it meets the outside face of their external wall. Draw a line down from there at 45 degrees towards your work. Anything of yours within 6 metres that reaches that line is caught. Within the first 3 metres, anything that reaches the line is already deeper than their foundations, so the 45 degree test adds the band from 3 to 6 metres.

Any kind of excavation can count. A foundation trench, a basement, a drainage trench or a soakaway can each trigger section 6. The booklet's notes to its example notice say a description such as "excavate to lay drainage/foundations" will do if the drawings clearly show what is proposed.

Examples

You may not know how deep the neighbour's foundations are. The Act does not require your notice to state it. It requires you to show the site and depth of your own excavation.

What the notice must contain

The notice referred to in subsection (5) shall be accompanied by plans and sections showing (a) the site and depth of any excavation the building owner proposes to make; (b) if he proposes to erect a building or structure, its site.

Section 6(5) adds that the notice must indicate your proposals and state whether you propose to underpin or otherwise strengthen or safeguard the foundations of the neighbour's building. The booklet also advises giving your name and address, the address of the building to be worked on, a description of the work and the start date, and dating the notice. Its example letter 8 is a template for this notice.

After service:

• The neighbour has 14 days to consent in writing. If they do not, a dispute is deemed to have arisen (section 6(7)) and the surveyor procedure in section 10 applies.
• You may, and if they require it you must, underpin or otherwise safeguard their foundations at your own expense, so far as necessary (section 6(3)).
• The notice ceases to have effect if the work has not begun within 12 months and is not prosecuted with due diligence (section 6(8)).
• When the work is finished, the neighbour can ask you for particulars, including plans and sections (section 6(9)).

The Act extends to England and Wales only (section 22(4)). Section 6 applies alongside any notice needed under section 1 or section 2 for the same project. See do you need a party wall agreement for those.

What this means for you

Whether section 6 applies is decided by your foundation design and the neighbour's building, so check it once your structural engineer has set foundation depths. Serve the notice at least one month before digging starts, with the plans and sections the Act requires.

Parallel does not act as a party wall surveyor. On projects we design, we prepare the drawings that accompany the notices, including the plans and sections a section 6 notice needs.