Japanese knotweed – Breach of duty of care – Causation – residual diminution in value
The facts
The claim arose from the encroachment of Japanese knotweed from the appellant local authority's (“LA”) land into the respondent's garden. The respondent had purchased his property in 2004, after the encroachment of Japanese knotweed had begun. It was held that the LA could not be blamed for the encroachment at this time. An actionable tort of private nuisance however arose in 2013, when the local authority should have become aware of the risk of damage and loss of amenity to the respondent's land as a result of information about Japanese knotweed which became publicly available at that time. The LA was held liable in private nuisance for having failed to prevent the encroachment between 2013 to 2018 when the LA implemented a reasonable and effective treatment programme.
The Japanese knotweed could be treated with herbicide, which would kill the above-ground growth of the weed but leave below-ground Japanese knotweed rhizomes remaining in the garden's soil. The respondent conceded that he could not claim the costs of the herbicide treatment, since the treatment would have been required in any event because of the non-tortious encroachment before 2013. Damages were however claimed for the residual diminution in his property's value which would persist after the completion of the treatment because of the continuing presence of Japanese knotweed rhizomes in the garden's soil resulting in continuing stigma in the eyes of prospective purchasers of the property.
His claim was dismissed by the district judge as being one for pure economic loss. The Court of Appeal overturned the decision on the basis of Delaware Mansions Ltd v Westminster City Council [2001] UKHL 55 holding that the LA's continuing and undisputed breach of duty between 2013 and 2018 caused harm consisting of the residual diminution in value which persisted at the end of the period of continuing nuisance; therefore, it was harm caused by the continuing breach. Judgment was entered in favour of the respondent in the sum of £4,900.
Judgment
The Supreme Court allowed the appeal. The Supreme Court held that the Court of Appeal had misunderstood Delaware. Properly analysed the decision in Delaware was that a claimant is entitled to recover the reasonable costs incurred in abating a continuing nuisance. In this case the loss representing the diminution in the property's value was not an aspect of reasonable costs incurred in abating a continuing nuisance. Delaware was therefore not applicable.
The applicable test was the conventional “but for” test. The question was whether the diminution in value had occurred “but for” the LA's breach of duty between 2013 and 2018. If that diminution would have occurred in any event, then there would be no causal link, as a matter of factual causation, between the breach of duty and the diminution in value.
The answer to the “but for” test in this case had to be seen in the context that there was no evidence and no finding by the district judge that the LA's breach of duty between 2013 and 2018 had increased or materially contributed to the diminution in value of the respondent's land. The evidence was that the diminution had occurred long before any breach by the LA of the duty in private nuisance first occurred. Accordingly, the application of the “but for” test in this case eliminated the LA's subsequent breach of duty as a causative factor. The fact that the diminution in value would have occurred in any event meant that there was no causal link between the breach of duty and the diminution in value claimed.
Lesson learnt
The case confirms that the conventional ‘but for' test for causation applies to causes of action in private nuisance, including those arising in respect of natural hazards and those involving continuing nuisances. This clarification on causation is to be welcomed. It follows that to successfully claim damages in this type of case a claimant will have to plead and evidence that additional or separate diminution in value has been caused after the initial encroachment by a delay in effective treating of the knotweed. This may not be straightforward.
The Supreme Court however expressly declined to determine three issues:
- The question of whether the encroachment of Japanese knotweed is a continuing nuisance;
- The impact of recent scientific research, which suggests that Japanese knotweed's reputation as a pernicious plant is undeserved (or at least overstated), as set out in the RICS Guidance Note 2022 “Japanese knotweed and residential property”;
- The implications of Japanese knotweed having first encroached into the respondent's property before his purchase.
It is also interesting to note that the Supreme Court did again not deal with the thorny issue of limiting periods in this type of case. So watch this space.
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