In a word – no.

“De minimis” is a legal principle which allows for matters that are small scale or of insufficient importance to be exempted from a rule or requirement. It can be used by the courts as an exclusionary tool to dismiss trivial matters from litigation. It can be applied to any legal rules, and if disputed, a court may apply it as they see fit – entirely at the discretion of the judge. It is not necessary to serve a Party Wall Notice if the matter is “de minimis”, but when that principle applies is open to question. There is nothing written into the Party Wall Act giving any guidance, but the Government’s explanatory booklet gives the following example of what might be considered de minimis in the Party Wall Act context;

“Things like:

  • drilling into a party wall to fix plugs and screws for ordinary wall units or shelving
  • cutting into a party wall to add or replace recessed electric wiring and sockets
  • removing old plaster and replastering

may all be too minor to require a notice under the Act. However, the key point is whether your planned work might have any possible consequences for the structural strength and support functions of the party wall as a whole, or cause damage to the Adjoining Owner’s side of the wall.

This is useful guidance for homeowners on where the line might be drawn on some common building activities which might be undertaken to a party wall, and common sense can hopefully prevail in relation to other activities not listed. However, no examples are given when it comes to the issue of “Adjacent Excavations”. The Act says that a Notice must be served before excavating below the level of a neighbour’s foundations, within 3m of them, but what if the excavations are only fractionally deeper, and inconsequential? Do you still need to serve a Notice to the letter of the 3m parameter in the Act?

Also, what about when there are a number of flats next door? Do you need to serve a Notice on all of them? Some of them may well be far enough away that the risk to them is de minimis even if there is a risk to the closest one.

The same principle of risk of possible consequences as stated above in the Government publication must apply to all parts of the PWA including adjacent excavations (section 6) just as it does to cutting into a party wall (section 2). So, one should consider what the risk of your proposed excavations (or other work) actually is, before deciding whether the matter is “de minimis” or not. Of course, a Building Owner might regard it as “de minimis” whereas the Adjoining Owner does not, and a judge faced with an application for an injunction might be persuaded either way – depending on the strength of the arguments put before them. Indeed, even if the two neighbours had never heard of the principle of “de minimis”, a judge may still rule it so – particularly if they thought the injunction was being unnecessarily sought.

Homeowners should be very cautious about accepting advice to seek an injunction to stop their neighbour from carrying out such excavations (or minor work to a party wall) – particularly if a competent independent expert has advised that the risk to the adjacent property is insignificant.  Costs generally follow the decision in injunction proceedings so if you lose the case you may be faced with paying your own costs as well as those of your neighbour – which could be substantial if they bring their expert into court as a witness.

For the Building Owner, it would be advisable, before proceeding with excavations which you think are “de minimis” (but your neighbour does not agree are so), to seek advice from a suitable independent professional to state in writing how significant they think the risks are. If they consider that there is no significant risk, and the excavations are “de minimis” then perhaps the Adjoining Owner might be dissuaded from seeking an injunction, but if the professional points out risks that you had not foreseen, then you might decide it best to serve a Notice of Adjacent Excavations after all.

The 3m rule applies to any form of excavation below the level of the adjacent foundations, so in theory just digging a small shallow depression in the garden within 3m of a garage built off a shallow slab base would require Notice being served, but nobody would consider the serving of a Notice appropriate in that example. As would digging a trial pit to check on foundation depth (which inevitably means digger a little below the bottom of the foundation). Excavating a deep trench at exactly the same depth as an adjacent deep foundation would not trigger the Act, but technically if any part of that excavation were to go any deeper (even just 1mm) that would trigger the need for a Notice. Again, this would probably be regarded by most as “de minimis”.

So when would an excavation pose a risk high enough not to be “de minimis”?

This depends on various matters such as;

  • soil type – some soils have a much greater “angle of repose” (self-support) than others – so a collapse of the excavation sides might affect a greater area,
  • the length of time excavations are left open to the elements (particularly rainfall),
  • groundwater conditions
  • Ground strength and the load imposed by the new construction (could the weight of the new building cause consolidation of the ground beneath the neighbour’s foundation?)
  • Proximity of excavations to adjacent foundations
  • Type of adjacent foundations (a reinforced raft or slab would be much less vulnerable than an old weak clinker strip footing)
  • The length of the excavations within 3m – a trench parallel to an existing foundation is much more risky than the end of a trench which is perpendicular to that foundation.
  • Proposed method of excavation – machinery causing excessive vibration might pose a risk to a vulnerable adjacent building, even if there was no risk of landslip. But excavations down to the same level are not covered by the Act, it is only the bottom part of the excavation (which is below the adjacent foundation level) which is potentially covered, so if that final part of the trench could be excavated in a different way which did not cause vibrations, then the risk could perhaps be reduced below the “de minimis” threshold.

Generally speaking, as a starting point if a 45-degree line taken from the bottom of the neighbour’s foundation is not significantly cut by the excavation, then there may well not be much risk, but that is an over-simplification, and a host of factors should be given careful consideration by an appropriately experienced expert before determining the actual risk. Interestingly, the diagrams given in several well-respected texts show situations which in most soil types would not in fact, on the face of it, present much of a risk at all.

In most cases, the depth of the neighbour’s foundation is not known so should be assumed based on building age / type / subsoil and Building Regulations in force at the time of construction. It would be wise for a Building Owner to make a conservative assumption or (in some cases) offer to establish the depth for certain by making a small trial pit excavation perhaps – with landscaping suitably made good afterwards of course!

Notices of Adjacent Excavations were introduced as a requirement of the London Building Acts (Amendment) Act 1939 (as “10-foot” and “20-foot” Notices), which applied only to inner London, where major developments were being undertaken using heavy machinery in very close proximity to older buildings, and risks were much higher than the typical modern provincial back garden extension situation. They were never intended to apply to trivial matters of insignificant risk. They were subsequently imported into the current 1996 Act (re-named 3m and 6m notices) and now apply to the whole country, including many cases where risks are in fact non-existent.

The property profession is used to serving these Notices in all cases which fall within the 3m rule without even considering the actual risk, or whether the principle of “de minimis” could / should apply. Consequently there may be found to be resistance from some, to the idea that Notices of Adjacent Excavation may not always be appropriate or necessary. This is often based on the view that an Adjoining Owner is being “cheated” out of their rights under the PWA by their neighbour undertaking excavations without having served a Notice (a recent Court of Appeal decision held that the Act cannot apply unless notice is served). However, in cases where the risk has been carefully assessed and deemed not significant, it could well be perfectly reasonable and acceptable that no Notice is served based on the “de minimis” principle. Would a homeowner feel cheated and seek an injunction to stop their neighbour from putting up bookshelves on the party wall? Would he feel cheated if his neighbour was digging his garden within 3m of the garage at a depth slightly beneath the slab? Very unlikley. The same principal applies to excavations for foundations which pose insignificant risk.

It would still make good sense for the two neighbours to make a record of any pre-existing damage (either themselves or through an agreed third party – such as a surveyor or contractor) before work starts – so as to avoid any later dispute over damage found. The common law still applies if no Notice is served, so if later damage were found to be caused by the work being undertaken negligently, the neighbour would still have a right to pursue a common law claim.

One good reason for serving a Notice though – even when the risk is deemed insignificant – might be that the Building owner’s wall is very close to the boundary and they want access to the neighbour’s land in order to place scaffolding or undertake work from the neighbour’s side (but the neighbour is unwilling to allow that). Using the PWA can give such access despite the neighbour’s objections.

Sensible discussions between neighbouring owners (backed up by appropriate expert advice if necessary) should be able to avoid unnecessary expense in party wall surveyors’ fees and bad feeling between neighbours.