What is the connection between a tree survey for house purchase, a strange instrument and the curious case of Kahn and Kahn v Helen Sheila Kane

I suppose that if you were to purchase a new house, the last thing you would be interested in would be a tree survey. Actually I propose that it should be the first thing you should think about! With the mentioned case precedent, a 10m Lawson Cypress hedge adjacent to a neighbouring property was blamed for causing subsidence. However Mrs Kane asserted that such damage was not reasonably foreseeable. It was found that “reasonably foreseeable” was an objective test as to what a reasonable person should have known, in this case a reasonably prudent landowner. In this case subjective knowledge increased liability. The Judge found that: “In my judgment, the purpose of the standard being set by the knowledge imputed to a class of persons is to impose a higher standard on persons in that class. It therefore creates a floor but not a ceiling on the level of knowledge so that subjective knowledge can raise the standard. However, lack of actual knowledge cannot lower the standard or exclude liability which would be imposed based on the standard generally imposed.” Thus without subjective knowledge there is still a floor for liability on the basisi of objective knowledge. Thus liability for tree root subsidence can be established against a private property owner regardless of lack of subjective knowledge. Thus when purchasing a property it would be key to establish if there is any potential liability for indirect influence of root moisture uptake on shrinkable clay subsoils. The Atterberg unit (the strange instrument!) is key in this regards in assessing what is called the “plasticity index” of clay soils. It is a high level of plasticity index where risk of subsidence is present. When acquiring an arboricultural report for property purchases, I propose that it should be considered mandatory that where clay soils are present, at least a preliminary plasticity assessment should be made to identify potential liability to neighbouring property. This is why IROS possesses the instrumentation needed to assess plasticity index for clay soils.

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