Party wall notice for a basement: the 3m and 6m rules

When a basement dig is notifiable

Section 6 of the Party Wall etc. Act 1996 is titled "adjacent excavation and construction", and it sets out two triggers. Government guidance summarises the Act as covering "excavation near to and below the foundation level of neighbouring buildings" (gov.uk); the statute itself is more precise.

The 3 metre rule, section 6(1)

It applies where 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 any part of the proposed excavation will within those three metres "extend to a lower level than the level of the bottom of the foundations" of the neighbour's building (section 6).

The 6 metre rule, section 6(2)

It also applies within six metres, where any part of the proposed excavation "will within those six metres meet a plane drawn downwards in the direction of the excavation ... at an angle of forty-five degrees to the horizontal" from the bottom of the neighbour's foundations. In practice: the deeper you dig, the further away the rule reaches.

For a basement under a terraced or semi-detached house, one or both will almost always apply. Note that section 6(4) deems owners within those distances to be adjoining owners for the purposes of the section, so "we are not attached" is not the test. Distance and depth are.

The notice

• Section 6(5) requires the building owner to serve notice at least one month before beginning to excavate.
• The notice must state the proposal and whether you intend to underpin or otherwise strengthen the neighbour's foundations, and it is accompanied by plans and sections showing the site and depth of the excavation and, where you propose to erect a building or structure, its site.
• Serve every adjoining owner whose building falls inside the distances, freeholders and long leaseholders alike. A block of flats next door can mean several notices.
• Notices under the Act are valid for a limited period, so do not serve years in advance of a start on site.

The duty that surprises people

Section 6(3) provides that the building owner "may, and if required by the adjoining owner shall, at his own expense underpin or otherwise strengthen or safeguard the foundations of the building or structure of the adjoining owner so far as may be necessary". In other words, your neighbour can require you to pay for protecting their foundations. On a deep dig next to an old shallow-founded terrace that is not a footnote, it is a budget line, and it is one of the reasons basement quotes should state clearly whether such works are included.

How it plays out in practice

1. Serve valid notices with plans and sections, at least one month before excavation begins.
2. If every adjoining owner consents in writing, no award is needed, though a schedule of condition of their property protects both sides.
3. If an owner dissents, or does not respond, surveyors are appointed and produce a party wall award setting out how the works may proceed, usually with a schedule of condition and conditions on monitoring and working hours.
4. Budget for surveyor fees as a separate line: for basement schemes these are typically higher than for a simple loft or extension because of the engineering detail involved.
5. Keep the structural engineer and the party wall surveyor talking. Most delay on basement schemes comes from the award waiting on temporary works information.

This page quotes the Act as it stands and links the section text so you can read it yourself. It is not legal advice. Party wall matters on basements are technical and adversarial when they go wrong, so appoint a surveyor experienced in basement work rather than the cheapest available.