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Legal case

Supreme Court hands down Judgment unanimously overturning landmark court ruling in Japanese Knotweed claim

An interesting case last month for property insurers following the Supreme Court’s decision in Davis v Bridgend County Borough Council [2024] UKSC 15

On 8 May 2024, the Supreme Court unanimously overturned the Court of Appeal decision in Davis v Bridgend County Borough Council [2024] UKSC 15, thus clarifying the basis on which diminution in value may or may not be awarded in Japanese Knotweed claims.

What is Japanese Knotweed?

Japanese Knotweed (Reynoutria japonica syn. Fallopia japonica) is a fast-growing, rapidly-spreading and strong clump-forming invasive perennial weed that is native to Japan, Taiwan and Northern China. It was first introduced to the UK in the early 19th Century as an ornamental plant.

Why is it a problem?

Japanese Knotweed is just about indestructible. Its roots are known to grow up to three metres down into the ground and seven metres across and it can grow through concrete, tarmac and brickwork. It can therefore cause significant damage to buildings, roads and structures, which presents issues for current and future property owners.

Background to the claim

In 2004, Mr Davies purchased his property at 10 Dinham Street, Bridgend (“the Property”). Beyond the back garden to the Property was an embankment leading down to a cycle path and an old railway line, owned by Bridgend County Borough Council (“the Council”). Unbeknownst to Mr Davies, Japanese Knotweed had encroached from the Council’s land to his land underground.

In 2012 a Royal Institute of Chartered Surveyors (RICS) report was published on Japanese Knotweed describing the difficulties it can cause.

In 2017, some 13 years later, Mr Davies discovered that the Japanese Knotweed at the Property might pose a problem if he were to sell the Property. Mr Davies therefore brought proceedings against the Council seeking damages, alleging a breach of duty under the tort of nuisance, arising from the encroachment of Japanese Knotweed onto his land.

First instance decision

The District Judge found that it was very probable that the encroachment of the Knotweed from the Council’s land onto the Property began well before 2004. However, the Council could not be blamed for the encroachment before 2013, as it was not actionable as private nuisance.

An action in private nuisance later arose in 2012 when the Council became, or ought to have become, aware of the risk of damage and loss of amenity to the land purchased by Mr Davies.

The Council was held liable in private nuisance for having failed to implement a reasonable and effective treatment programme to control the Knotweed and prevent the encroachment in the period of 2013 to 2018.

Mr Davies was awarded damages for residual diminution in the value of the Property. 

Court of Appeal decision

The Council argued against the claim for residual diminution in value on the basis that ‘but for’ its breach of duty from 2013 to 2018, the value of the Property would have still been diminished because of the earlier encroachment.

The Court of Appeal rejected the Council’s argument on causation and followed the House of Lords’ decision in Delaware Mansions Ltd v Westminster City Council [2001] UKHL 55. It held that due to the Council’s ongoing breach of duty, the diminution in value was continuing and harm could therefore be said to have been caused by the breach of duty. This determined that the Council was liable for the residual diminution in value which had occurred prior to the breach arising.

The Court of Appeal’s decision was problematic as it awarded damages where the breach was not causative of the loss. Many previous Japanese Knotweed claims had been challenged on the basis that losses which proceeded breach could not be recoverable.

Supreme Court decision

The Supreme Court unanimously overturned the Court of Appeal’s decision on the basis that Delaware had been misunderstood. The decision in Delaware was that a claimant is entitled to recover the reasonable costs incurred in abating a continuing nuisance. In Davies the loss representing the diminution in the property’s value was not an aspect of reasonable costs incurred in abating a continuing nuisance and so, the decision in Delaware was not applicable.

The Supreme Court accepted the Defendant’s causation argument that the basic principles of ‘but for’ should apply and held that residual diminution in value of the property which has occurred prior to the Council’s breach did not form part of any award.

In his leading judgment, Lord Stephens explained the key decision as follows:

“If the diminution in value would have occurred in any event, then the defendant’s breach of duty is eliminated as a cause of the diminution in value so that there would be no causal link between the defendant’s breach of duty and the diminution in value.”

Further clarification was provided by Lord Burrows:

“The JKW was already present on the claimant’s land before 2013 so that the residual diminution in value had already been brought about by the natural, non-actionable, encroachment of the JKW […] the claimant has failed to prove that the alleged damage (or, put another way, the alleged undue interference with the claimant’s use and enjoyment of land) was factually caused by the breach of duty which was committed from 2013.”

In summary, the encroachment and resultant diminution had occurred long before any breach by the Council, whose subsequent breach could therefore be eliminated as a causative factor.


The Supreme Court’s decision on causation is a logical and common-sense conclusion which has provided some much anticipated clarity for Japanese Knotweed claims going forward.

The decision will be welcomed by landowners as it means that the damage they may be liable for is likely to be limited to those losses that are identifiably caused by the breach of duty.

Insurers will also welcome the likely impact that this decision will have on claimants’ appetite for pursuing litigation in this area.


Faye Sanderson

0161 233 1292

Faye has particular expertise in property damage, construction and product liability related disputes, and regularly acts for insurers and policyholders in such claims.

Maisie Heeley

Trainee Solicitor
0161 214 0674

Maisie is a Trainee Solicitor currently working in the Property Damage Team.


The UK Property Damage Team at Weightmans comprises fee earners at every level who, between them, have decades of experience in all aspects of property and business interruption insurance.

Whatever the size, nature or location of your activities, we can help you to manage and resolve claims efficiently and effectively for the best possible outcome. For more information on our services, please contact our property damage team.

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