The Supreme Court has handed down judgment in the case of Davies v Bridgend County Borough Council [2023] UKSC 16. They have overturned the decision of the Court of Appeal that the defendant was responsible for residual diminution in value of the claimant’s land following treatment of Japanese knotweed even where the spread had occurred before breach.

Matthew represented the defendant in the Supreme Court and was instructed by Dolmans Solicitors.

Key points at a glance

  • If Japanese knotweed spread from a defendant’s land to a claimant’s land before the defendant was in breach of duty, then the defendant is not responsible for either (a) the cost of treating knotweed on the claimant’s land; or (b) the residual post-treatment diminution in value of the claimant’s land. That is, in cases of historic spread of Japanese knotweed, defendants have a good causation defence.
  • By analogy, if the spread was more than 6 years before issue, the loss arose outside of the limitation period, and a defendant is not responsible for the need for treatment or the residual post-treatment diminution in value. That is, if the Japanese knotweed spread more than 6 years before issue (i.e. as of claims issued now, if the spread was before 2018), defendants have a good defence.
  • It is for a claimant to prove that breach caused loss.
  • Delaware Mansions v Westminster City Council [2002]1 AC 321 (“Delaware”) is properly interpreted as determining that a claimant landowner is entitled to recover the reasonable costs of abating a continuing nuisance


The claimant owned a terraced house with a garden at the back in South Wales. Beyond his back garden wall was an embankment leading down to a cycle path on an old railway line. As is common on rail corridors, there was Japanese knotweed by the cycle path at the bottom of the embankment, but the evidence was that Japanese knotweed at the top of the embankment had probably not grown up from the bottom. Rather it had probably been dumped over the garden wall by one of the residents. There it had grown, unnoticed by the defendant. Some time before 2004, when the claimant bought his property, the knotweed had encroached from the defendant’s land, underground, such that there were rhizomes on the claimant’s land before 2004.

He first became aware that Japanese knotweed might be a problem in 2017. He made no attempt to find out who owned the land beyond the end of his garden until someone knocked on his door to tell him that he had Japanese knotweed on his property, and that they could represent him in a claim. A letter of claim was sent in 2019.

The case followed what has become a fairly ordinary pattern: the claimant chose its preferred Japanese knotweed and valuation experts, and instructed them unilaterally. Somewhat unusually the claimant decided against using the first 2 experts from whom he obtained reports, and replaced them, again unilaterally, with 2 new experts. At case management stage the court, in the usual way, determined that the value of the claim, which was put at £10,000 to £40,000 on the claim form and about £34,000 in the Particulars of Claim, although the realistic value of what was claimed was closer to £12,000, did not justify separate experts for the parties and required a new single joint Japanese knotweed expert. The defendant lived with the claimant’s (second) valuer.

Just before trial the defendant found and disclosed previously undiscovered documents which suggested that it knew about the knotweed well before its evidence showed that it started treating it, in 2018. District Judge Fouracre followed the approach in Williams & Waistell v Network Rail Infrastructure Limited [2019] QB 601, and held that date of knowledge of a foreseeable risk of harm was after publication of the Royal Institution of Chartered Surveyors paper in 2012, and that from 2013 the defendant ought to have been treating the knotweed. It was held to be in breach of duty from 2013 to 2018. It proved treatment of the Japanese knotweed on its land from 2018. Accordingly there was an actionable and continuing nuisance from 2013 to 2018.

The first instance decision on damages

A claim for general damages for distress and inconvenience was dismissed. The judge rejected the claimant’s evidence that he was ‘immensely distressed’ by the presence of knotweed on his land, noting that from when he first became aware that Japanese knotweed might cause a problem – no earlier than 2017 – he did nothing to discover who owned the land at the end of his garden or contact the defendant.

That left the claim for diminution in value of the property which was made up of various elements, dealt with as follows:

  • Cost of treatment. This was claimed at £3,600 on the basis of the claimant’s expert upon whom he did not have permission to rely. The single joint expert gave a figure of £1,800. The defendant argued that it was always going to be necessary to spend that money, even before breach, because the knotweed had spread before breach. The claimant accepted that argument and conceded that this sum was irrecoverable (a concession which, incidentally, the Supreme Court considered properly made).
  • Disturbance and inconvenience. The claimant claimed £1,200 for the inconvenience of having knotweed treatment. This faced the problems that there was no knotweed to see on the claimant’s land by the time of the single joint expert’s inspection, and also that treatment was always going to be required regardless of breach. It was not pursued.
  • Neighbour cooperation. The sum of £1,400 was said to reflect the need to secure cooperation from the neighbours in treating the knotweed. Since the relevant neighbour was the defendant, who was actively treating the knotweed – therefore obviously cooperating – this was held to be irrecoverable.
  • Temporary loss of land. This was claimed at £1,000, but since there was no visible knotweed in the garden by the time of the single joint expert’s inspection, it was impossible to say that there would be temporary loss of use of the land, so this was rejected.

That left the claim for residual diminution in value after treatment. Whilst claims are sometimes put on the basis of diminution in value ignoring the effect of treatment, the correct measure of loss, if recoverable, would be cost of treatment and the residual diminution in value. That residual diminution arises due to an enduring stigma or ‘blight’ associated with Japanese knotweed.

District Judge Fouracre found, and this was upheld by HHJ Beard on first appeal, that the residual diminution in value was irrecoverable because it was pure economic loss, and the tort of nuisance did not exist to protect economic interests.

The Court of Appeal on diminution

The Court of Appeal overturned the decision on residual diminution. They held that the ratio of Williams is that there is no nuisance in the absence of encroachment of rhizomes merely because having Japanese knotweed next-door reduces the value of a claimant’s property. To hold otherwise would be to allow a claim for pure economic loss. However, if there has been encroachment, there has been physical interference with the claimant’s property, and consequential losses, including diminution in value, are recoverable.

That part of the decision of the Court of Appeal was not appealed further to the Supreme Court. Rather the Supreme Court decision relates to causation.

The Court of Appeal on causation 

Whilst the claimant accepted that the treatment cost was always going to be necessary (regardless of breach), he also contended that the residual diminution was recoverable. The nuisance was a continuing nuisance. The defendant argued that but for the breach the claimant would have had a property affected by (value diminished by) knotweed, and given the breach he has a property affected by (value diminished by) knotweed, such that the breach had made no difference. Put another way, the rhizomes had, on the evidence, spread by 2004, so it made no difference that the defendant failed to treat the knotweed from 2013 to 2018: the problem had arisen before the breach. The defendant’s proposition was that loss which precedes breach cannot have been caused by the breach.

The Court of Appeal rejected that argument. They drew an analogy with Delaware Mansions [2002]1 AC 321. In that case tree-roots caused damage to a property in 1989, and the property was sold to new owners in 1990. The new owners spent over £1/2m on underpinning and sued the tree-owner. At first instance the claim failed on the basis that the damage was said to have occurred during the original owner’s ownership. The Court of Appeal and House of Lords disagreed with that decision, holding that the cost of underpinning arising from the tort could be recovered by the owner who had to incur the cost.

In Davies Birss LJ summarised this (judgment paragraph 47) as “The fact the encroachment was historic was no answer when there was a continuing breach of duty as a result of persisting encroachment.

The Supreme Court on causation

The defendant’s argument has always been simple: if loss preceded breach then breach cannot have caused loss, and since we are concerned with the residual (post-treatment) diminution in value of the claimant’s land, that loss had arisen before breach. But for breach the claimant would have had a property that required treatment of Japanese knotweed and was then property that once had knotweed growing on it; given the breach the claimant was in the same position, with a property that required treatment of Japanese knotweed and was then property that once had knotweed growing on it.

That is, the Supreme Court has agreed with the defendant’s argument.

As a matter of logic the same argument applies in relation to limitation too. Davies was argued on the basis that Japanese knotweed is a continuing nuisance. Since Jalla v Shell [2023] UKSC 16; [2023]2 WLR 1085, that is open to doubt, and the intervener in Davies positively contended that JKW is not a continuing nuisance. Whether Japanese knotweed is properly seen, on the facts of any given case, as a continuing nuisance, likely by further encroachment of rhizomes from a defendant’s land to a claimant’s land, ought not impact on the limitation defence. Encroachment over 6 years before the claim was brought is statute barred whether the nuisance was a continuing one after the encroachment or not. That argument will probably see off more Japanese knotweed claims than a determination that the encroachment probably occurred before the date of legal knowledge (which, for large/ institutional landowners, is generally taken as 2012/13 following Williams). For a claim issued now, in 2024, the encroachment would need to be 2018 or later.

It is also worth particularly noting the Supreme Court’s explanation of Delaware. The reasoning of the House of Lords in that case could perhaps have been clearer. It has been clarified that the reason why the new owner could recover the cost of underpinning was because a claimant can recover the reasonable costs of abating a continuing nuisance. In Delaware itself, Lord Cooke of Thorndon (at paragraph 33) suggested that a potential claimant ought to give a potential defendant the opportunity to deal with an ongoing nuisance before becoming entitled to the cost of abatement.

What next

Looking online, it is possible to find some rather scathing comments about claimant solicitors running Japanese knotweed claims. My experience is that claimant solicitors’ costs bills are very significantly larger than the sums at stake in the litigation. The new fixed costs rules for sub-£100,000 cases will stop some of the excess, but costs are still likely to be a significant part of the landscape in these claims. That makes me think that claims might well continue. I’m a little reluctant to make predictions for the future, but my best guesses are:

  • I suspect that claimant firms will continue to wish to use Japanese knotweed experts who tell then what they want to hear. What they will want to hear now is that the Japanese knotweed spread within the last 6 years, to avoid the limitation problem. I’ve already noticed a change in the default position of regular claimant experts: back in 2020/21 I expected reports to say that the knotweed rhizomes had probably spread to a claimant’s land long ago, thereby suggestive of greater (longer) culpability. When defendants started running the argument in Davies, claimant experts’ default position seemed to change to saying that knotweed had been present for a relatively short time. I will not be surprised to see more of that.
  • I therefore expect that we will continue to have the same problematic battle in which defendants justifiably do not accept the expert selected by the claimant, and ask the court on allocation to appoint more balanced single joint knotweed expert. My experience is that that request is almost always acceded to, it being the proportionate way to deal with these claims.
  • Perhaps claimants will seek to prove that breach has increased If knotweed encroached before breach (or more than 6 years before issue), that will have given rise to a treatment cost and a post-treatment residual diminution in value, and that which arose before breach, or more than 6 years before issue, will not be recoverable. Perhaps, however, a claimant could show that there is more treatment cost or more residual diminution in value arising as a result of the period of breach. Such argument will face evidential difficulties, but is theoretically available.

Whilst this case is undoubtedly a significant win for defendants, they should not see it as a justification for taking Japanese knotweed less seriously. It remains a significant problem. Landowners should, as before, think about what steps they need to take to meet their legal duty to take reasonable care for their neighbours.

Matthew White

8th May 2024

Matthew represented the defendant in the Supreme Court. He advises and deals with claims relating to the spread of Japanese knotweed for defendants, particularly local authorities, housing associations, and bodies with large land holdings. Other members of St John’s Chambers deal with claims relating to the spread of Japanese knotweed for claimants and defendants. Our Real Estate team is pre-eminent in the area of real estate litigation on the Western Circuit and in Wales, and deals with all areas of real estate law.