The 3m test
Section 6 applies where the excavation is within a horizontal distance of 3m from any part of the adjoining owner's building or structure and goes below the level of the bottom of its foundations. The measurement is not simply 3m from the fence or title boundary. [1]
As a practical example, the closest relevant structure may be the neighbour's rear extension rather than the main house. A project described as a garden extension can therefore require a different assessment from one based only on the distance between the original houses. [1]
The 6m test
For excavation within 6m, the further test concerns a plane drawn downwards at 45 degrees from the bottom of the adjoining foundations, beginning at the outside face of the external wall. The excavation must meet that plane for this limb of section 6 to apply. [1]
This is a three-dimensional test, not a general 6m exclusion zone. It is also different from planning guidance which may use angles to assess daylight or the effect of an extension on neighbouring windows. The two assessments answer different questions. [1][2]
What information is needed?
Prepare a drawing that distinguishes the following:
- the neighbouring building or structure being assessed;
- the horizontal distance to the excavation;
- the bottom level of the neighbouring foundations;
- the proposed excavation position and depth, including relevant changes in level. [1]
Where existing foundation information is uncertain, do not fill the gap with a presumed standard depth. Ask the project engineer and party wall adviser how the information will be established and how any uncertainty affects the design and notice. Government guidance expects the excavation notice to explain the proposed safeguarding of adjoining foundations. [3]
A detached house is not automatically outside these rules. Section 6 is concerned with nearby buildings and structures, not only buildings sharing a wall. Conversely, not every excavation within the stated distances requires notice where the depth test is not met. [1]
Notice and response
The normal minimum notice period is one month before excavation. The notice must include plans and sections showing the site and depth of the excavation and any proposed building or structure, and state whether adjoining foundations will be underpinned, strengthened or safeguarded. [1]
If the adjoining owner does not consent in writing within 14 days, a dispute is deemed to arise and the surveyor procedure is used. Do not confuse the response deadline with the earliest lawful start date. [1][4]
Recheck when the design changes
A deeper foundation, a revised basement layout or a changed excavation line can alter the assessment. Refer changes back to the engineer and appointed surveyor before the contractor follows revised drawings. Planning approval does not replace the Act's procedure or the building owner's duty to compensate for loss or damage resulting from qualifying work. [3][5]