The story so far – diminution in value

In the lower courts, Mr Davies successfully argued that he was entitled to damages for diminution in the value of his house caused by the spread of knotweed from the Council’s neighbouring land, even after it had been treated.

As we explained in our previous article on this case the Court of Appeal considered that Mr Davies’ losses stemmed from the continuing nuisance caused by the Japanese knotweed, so were recoverable.

While there was less than £5000 at stake in the case, the issues caused by Japanese knotweed for landowners such as local councils are potentially huge, which is why the dispute went all the way to the Supreme Court.

Issue before the Supreme Court – timing

The Supreme Court’s judgment today came down to timing – specifically, whether Mr Davies could claim damages for the encroachment of knotweed onto his land even though, at the time when the encroachment occurred, the Council was not under a duty to treat it.

The parties all agreed that the encroachment of the Japanese knotweed from the Council’s land had occurred long before 2004, when Mr Davies bought his house.

However, the presence of knotweed only amounted to an actionable nuisance from 2013 – when the Council should have become aware of the potential dangers of the knotweed spreading to neighbouring land and therefore had a duty to treat it – until 2018, when they actually did treat it.

The Council did not challenge Mr Davies’ right in principle to claim damages for the diminution in the value of his property, but argued that the loss was not caused by them committing a nuisance, because the loss pre-dated their duty to treat the knotweed.

Factual causation

In order to recover damages for nuisance, you need to demonstrate that “but for” the defendant’s actions, you would not have suffered any loss.

The Court of Appeal found that, because the presence of Japanese knotweed on Mr Davies’ land between 2013 and 2018 was a continuing nuisance, the Council was in breach of its duty to treat the knotweed during that period, and this caused Mr Davies’ loss.

The Supreme Court said the “but for” test in these circumstances required Mr Davies to demonstrate that he would not have suffered a loss “but for” the Council’s breach of duty  between 2013 and 2018. If he would have suffered a loss anyway, he could not demonstrate that the Council’s breach of duty caused that loss.

In applying that test, the Supreme Court found that there was no evidence that the Council’s inaction between 2013 and 2018 had increased or contributed to the diminution in value of Mr Davies’ land; rather, the diminution in value had happened back in 2004, long before the Council’s breach of duty arose.

This effectively removed the causal link between Mr Davies’ loss, which would have happened anyway, and the Council’s later breach of duty.

The Supreme Court, therefore, found in favour of the Council, and Mr Davies was not entitled to recover the diminution in the value of his property.

Interestingly, Mr Davies never claimed the costs of treating the Japanese knotweed from the Council, as he acknowledged that its presence was not caused by the Council’s breach. The Supreme Court decision extends this logic to the diminution in value claim.

Would the damage have been less if the Council had treated the knotweed earlier?

Mr Davies also sought to argue that the “blight” of knotweed decreased over time, so the loss in value of his house would have been lower if the Council had treated the knotweed in 2013, and he should be allowed to claim the difference.

The Supreme Court gave this argument short shrift on the basis that it had not been pleaded, there was no evidence to support it, and no actual figure had been claimed.

What does this mean for Japanese knotweed claims, and the law of nuisance?

The Council in this case did not challenge Mr Davies’ right to claim in nuisance for the diminution in value of his property caused by the presence of Japanese knotweed even after treatment – the case was a more nuanced consideration of causation arguments in unusual circumstances where the damage occurred before the duty to treat it arose. As the Supreme Court said, “that concept has rarely been the focus of attention in cases on the tort of private nuisance”.

The unusual timings in this case may be less relevant in future cases – one of the main reasons that the Council’s duty to treat the Japanese knotweed was said to arise in 2013 was because the RICS issued an information paper on Japanese knotweed in 2012, emphasising the dangers to neighbouring land.

The RICS issued an updated information paper in 2022, which revised its advice on Japanese knotweed, and noted that the risk of damage to neighbouring property, and the effect on value, had previously been overstated.

The RICS’ revised paper was not relevant in this case, as the lower courts’ decisions predated it. However, it may be relevant for future judgments, and could have a bearing on the nature of landowners’ duties, and the diminution in property values as a result of any breach.

Despite the quirky timings, this case is a valuable reminder that you can only claim in nuisance for losses which were caused by another person’s breach of duty, rather than the mere presence of something on your land, which has ramifications far beyond Japanese knotweed, and shows the difficulty of tracing a causal link where there is a continuing nuisance.