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An invasion of Japanese Knotweed claims

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By Polly McBride


Published 18 October 2018


As we have already commented on the Court of Appeal's decision in the Network Rail case, a detailed analysis is unnecessary.  If you wish to see that please click here to go to our article of 23 July 2018.

Save to say, the Claimants' houses were plagued by knotweed from a railway embankment at the foot of their gardens and succeeded in establishing a private nuisance as a result of an apparent inability on Network Rail's part to prevent an ongoing encroachment. 

The first instance decision, awarding the Claimants £10,500 and £10,000 respectively for residual diminution (essentially the risk of knotweed, after treatment, returning), was upheld by the Court of Appeal, but only because the appellate court refused to permit Network Rail to essentially introduce new expert evidence to argue that knotweed had been present in the area for many years.  There was, in fact, no challenge to the principle of awarding 'stigma' damages in nuisance claims.

What is of interest, in the claims against surveyors that are being made citing this case as a basis for diminution in value losses, is that in a large number of claims we are seeing, there is often no expert evidence on the issue of whether the knotweed could and therefore should have been identified by the surveyor at the time of original inspection.

Expert evidence tends to be limited to asserting that knotweed must have been present at the property at the time of inspection, rather than whether it should have been spotted by a reasonably competent surveyor, (which is, of course, the test for negligence).

Many claims also appear to be highly over-inflated on the basis that a property with knotweed is not mortgageable and therefore blighted. This is simply incorrect. Lenders will usually not have a problem, as long as an insurance backed treatment plan is in place.  Such plans usually cost in the region of £4,000 - £7,000 for 1-2 sq. m of knotweed, backed by 5 years' insurance.

Claims of failing to advise on the presence of knotweed are usually brought seeking not only these treatment costs but also diminution in value. We would suggest these are mutually exclusive concepts, not only is cost of repair not the correct measure but may not even have any material impact on value. On this topic the RICS advises 1:

"4.7  Impact on value

4.7.1 By quantifying the likely cost of treatment and any necessary repairs .., the impact of Japanese Knotweed can be taken into account in the valuation process and reflected in the same way as any other defect or item of disrepair.

4.7.3 If treatment is deemed necessary in cases where Japanese Knotweed is present in the grounds but no damage has been caused to the property itself, the sole expense may be the cost of the treatment itself. In some circumstances, this may have no adverse effect on value." (emphasis added)

Accordingly, while many of these claims seek damages on the basis that a property is un-mortgageable, and/or blighted by the mere presence (and potential return) of knotweed, in actual fact there may well be no diminution beyond the cost of treatment which, as mentioned above, is usually a relatively minor sum, assuming the knotweed has not caused any damage and covers a relatively small area.  And as Network Rail could have argued, had they only adduced the necessary evidence at first instance, if knotweed had been around the locality for long enough to pre-date the claimant's purchase, even that remedial cost might not have any impact on value.

We have also seen a number of claims that should really have been better brought against the vendor, in circumstances where there is clear evidence of the knotweed having been concealed; an altogether common tactic in our experience.

A surveyor does not, of course, have a duty to go digging around in the garden, so if knotweed has been cut down and/or covered, then it is quite possible that the plant was not visible to the surveyor carrying out a reasonably competent inspection (especially if outside the plant's peak growing period between May and September).  Additionally, where the inspection is carried out during the plant's dormant period (Autumn / Winter).


So what is the justification for the recent increase in claims; beyond pure opportunism?

A number of claim management companies and solicitors have sought to take advantage of the public's perceptions about knotweed and we are seeing a high number of claims that appear to have been made in the hope that at least a percentage of them will result in settlement.

Is this public fear justified?  Arguably not and there is an ever-growing consensus that knotweed is not as dangerous as widely believed and does not deserve its reputation.

This is in circumstances where effective treatment is widely available and relatively cheap and is, furthermore, backed by insurance. No one would argue that knotweed cannot be damaging, but the threat is often exaggerated and recent research suggests that, on its own, the plant will NOT damage buildings any more than other plant/tree interference.

Leeds University and global infrastructure services firm AECOM have just published a paper 2 setting out the findings of the most extensive research to date assessing the potential of knotweed to cause structural damage ,compared to other plants. The conclusions were that:

(i) there was no evidence to suggest that knotweed causes significant damage to buildings, even where growing in close proximity to a property (and certainly no more damage than other invasive species);
(ii) knotweed poses less of a risk to buildings than many woody species, particularly trees;
(iii) knotweed is capable of damaging buildings but this usually occurs due to an existing, inherent weakness or defect that has been exacerbated.

Unfortunately, when faced with the presence of knotweed in the garden, along with a history of negative and sensationalist press that has fuelled the panic and combined with promises from solicitors of a swift payout by insurers, we have the perfect storm for a potential flurry of claims.

Our advice to professionals is to ensure clear photographic evidence is taken of the garden (and adjoining properties where possible) when carrying out an inspection, particularly as this often gets overlooked in favour of the house itself. Also, if no obvious signs are evident say so in the report and if there is any uncertainty at all, recommend further specialist investigation.

We can only hope that public opinion will be calmed with more research and less sensationalist press over the coming months. Certainly with good contemporaneous photographic evidence and a robust defence which deploys all the above points, surveyors and their insurers would be well advised to stand firm in the face of such claims – frustration and a lack of return could be the best means of encouraging claim management companies to look elsewhere for their golden ticket.


[1] “Japanese Knotweed and Residential Property” – RICS professional information paper, 1st edition, 2012.

[2] AECOM/University of Leeds – "Japanese knotweed (Fallopia japonica): An analysis of capacity to cause structural damage (versus other plants) and typical rhizome extension" July 2018