The 3-Metre vs 6-Metre Excavation Tests Explained
Section 6 of the Party Wall etc. Act 1996 is the most frequently triggered section of the Act for domestic extensions. Many homeowners wrongly believe that because their planned extension sits 1 or 2 metres away from the boundary line and does not touch the shared wall, the Act does not apply. This is a severe legal error.
Section 6 is designed to protect neighbouring properties from ground subsidence, settlement, and heave caused by adjacent trench digging and soil extraction. It establishes two distinct geographic tests:
1. The 3-Metre Rule (Section 6(1)): If you propose to excavate or erect a building within 3 metres of any part of a building or structure belonging to an adjoining owner, and your excavation will penetrate to a lower level than the bottom of their foundations, you MUST serve a Section 6(1) Notice at least 1 month before digging.
Because older Victorian, Edwardian, and 1930s houses often sit on shallow corbelled brick foundations just 400mm–600mm deep, modern Building Regulations strip footings (typically 1,000mm–1,200mm deep) will almost always be deeper than your neighbour's foundations. Consequently, virtually all extensions within 3 metres require a Section 6 notice.
2. The 6-Metre Rule (Section 6(2)): If you propose to excavate within 6 metres of a neighbour's structure, and any part of the excavation cuts below a line projected downwards at an angle of 45 degrees to the horizontal from the bottom of their foundations, you MUST serve a Section 6(2) Notice at least 1 month prior.
The 6-metre rule typically catches deep basement excavations, stepped retaining walls, drainage shafts, swimming pools, and piled foundations.
| Statutory Rule | Distance From Neighbour's Wall | Depth Trigger | Notice Period Required |
|---|---|---|---|
| Section 6(1) | Within 3.0 metres (9.84 ft) | Deeper than base of neighbour's existing footings | 1 Month |
| Section 6(2) | Between 3.0 and 6.0 metres (19.68 ft) | Below 45-degree line from base of neighbour's footings | 1 Month |
| Beyond 6.0 Metres | Greater than 6.0 metres | No statutory party wall trigger (common law only) | None under 1996 Act |
Mandatory Engineering Drawing Requirements
Under Section 6(6), the notice is not legally served unless it is accompanied by drawings and sections.
Your notice must include:
• A scale site block plan showing the exact distance in millimetres between the proposed excavation trench and the neighbour's nearest external walls.
• A structural cross-section showing the depth of your proposed concrete footings or trench alongside the assumed or investigated foundation depth of the adjoining structure.
If your structural engineer has not yet finalised foundation depth calculations, you cannot legally serve a Section 6 notice.
Under Section 6(3), an adjoining owner has the statutory right to request that the building owner underpinning their adjoining wall at the building owner's expense if necessary to safeguard its stability. These technical details are formalized in the Party Wall Award.
Section 6 Compliance Checklist for Extensions
Follow this checklist before digging foundation trenches near a neighbouring home: