Need it done by a professional? THS Concepts carries out party wall monitoring for architects, engineers and contractors.
If you are digging foundations for an extension, building a basement or excavating close to a neighbouring building, section 6 of the Party Wall etc. Act 1996 may require you to serve notice on your neighbour before you start. Whether it applies comes down to two tests, usually called the 3 metre rule and the 6 metre rule. This checker works through both from three measurements and draws your excavation to scale.
Where an excavation is notifiable, the neighbour’s surveyor will often ask for the adjoining building to be monitored while the work goes on. Our party wall monitoring service provides the independent baseline and readings that the party wall award relies on.
Party wall 3 m and 6 m checker
Measure the distance horizontally. Measure both depths from the same ground level, ideally your own.
The 3 metre rule
Section 6(1) applies where you excavate, or excavate for and build, within 3 metres of a building or structure belonging to your neighbour, and any part of the work goes to a lower level than the bottom of that building’s foundations.
Both conditions have to be met. A dig 2 metres away that stays above the bottom of the neighbour’s foundations is not caught by the 3 metre rule. A dig 2 metres away that goes 100 mm below them is.
The 6 metre rule
Section 6(2) applies where you excavate, or excavate for and build, within 6 metres of the neighbour’s building or structure, and any part of the work meets a line drawn downwards at 45 degrees towards your excavation.
The 45 degree line starts where the level of the bottom of the neighbour’s foundations meets the outside face of their external wall. Because it falls one metre for every metre you move away, the line is 1 metre below the bottom of their foundations at 1 metre away, 3 metres below at 3 metres away and 6 metres below at the 6 metre limit. An excavation that reaches that line within 6 metres is notifiable. In practice the 6 metre rule catches deeper excavations, such as basements and piled or deep mass concrete foundations, that are further away.
The checker uses the nearest edge of your excavation, because that is where the 45 degree line is shallowest. If your excavation steps down as it moves away from the neighbour, check the deepest point at each distance as well.
The measurements you need
Distance. Measure horizontally from the nearest part of the neighbour’s building or structure to the nearest edge of your excavation. The Act measures from “any part of a building or structure”, so this is not the boundary line. Depending on the site it can be an extension, a garage, an outbuilding or a wall, and footings below ground can sit closer than the wall you can see. If you are not sure what counts, ask a party wall surveyor.
Depth of the neighbour’s foundations. The depth to the underside of their foundations, measured from the same level as your excavation depth. This is usually the hardest figure to get. A trial pit dug next to the wall is the most reliable source. Building control records, the original drawings or the engineer’s knowledge of similar houses in the street can also help. If you have to assume a depth, a shallower figure is the cautious choice, because it makes the rules more likely to apply.
Depth of your excavation. The deepest point of your dig near the neighbour, including the foundation itself, measured from the same level. If your site and the neighbour’s are at different levels, convert both depths to one level first. A topographical survey gives you the ground levels on both sides of the boundary and the exact position of the neighbouring building.
Worked examples
All three use a neighbour whose foundations are 0.9 m deep.
Rear extension, 2.5 m away, 1.2 m deep trench fill. The excavation is within 3 m and 0.3 m deeper than the neighbour’s foundations, so the 3 metre rule applies.
Basement, 4.5 m away, 3.5 m deep. The excavation is outside 3 m, so the 3 metre rule does not apply. At 4.5 m the 45 degree line is 0.9 + 4.5 = 5.4 m deep, and the basement stops at 3.5 m, so it does not reach the line. Section 6 does not apply on these figures.
Basement, 4 m away, 5.5 m deep. At 4 m the 45 degree line is 0.9 + 4.0 = 4.9 m deep. The basement goes to 5.5 m, so it crosses the line and the 6 metre rule applies.
What happens if your excavation is notifiable
You must serve notice on the adjoining owner at least one month before the work starts. The notice has to say whether you propose to underpin or otherwise strengthen or safeguard the foundations of their building, and must be accompanied by plans and sections showing where the excavation and any new building will be and how deep the excavation will go. Accurate section drawings of both buildings make this much easier for the surveyors to agree.
The adjoining owner can consent in writing within 14 days. If they do not reply within 14 days, or they dissent, a dispute is treated as having arisen. The usual route is for one agreed surveyor, or one surveyor for each owner, to make a party wall award setting out how and when the work is done. The adjoining owner can require you to underpin or safeguard their foundations so far as the work makes it necessary, at your expense. The notice lapses if the work has not started within 12 months.
Who pays the surveyors’ fees is decided in the award. Where the work is solely for the building owner’s benefit, the building owner generally pays.
Monitoring the neighbouring building
Awards for basements and deep excavations often require the neighbouring building to be monitored before and during the work. A baseline taken before any digging starts is the only way to show afterwards what moved and what was already there.
We provide basement excavation monitoring, crack monitoring of the adjoining property and vibration monitoring where piling or breaking out is involved, reported in a format that goes straight into the party wall file. If you are on the other side of the fence, read what to do when your neighbour is building a basement, and our guides to the signs of structural movement and how to monitor a crack. For typical prices, see structural monitoring costs.
What this checker does not cover
The checker deals with section 6 only. Building a new wall on or at the boundary line (section 1) and work to an existing party wall, such as cutting in steel beams or raising the wall (section 2), have their own notices and are not checked here. It also assumes that both depths are measured from one level and that your excavation has a single depth at its nearest edge. Unusual sites, sloping ground, sheet piling, underpinning schemes and anything close to a threshold need a party wall surveyor’s judgement.
Sources
Party Wall etc. Act 1996, section 6 on legislation.gov.uk, and the government’s Party Wall etc. Act 1996 explanatory booklet.