Complaints involving Japanese Knotweed ("JKW") have risen quicker than the plant itself, since a High Court decision earlier this year. This may be a temporary state of affairs, with the case due in the Court of Appeal early next year; but for now professionals need to be on their guard.

Williams & Waistell v Network Rail, brought as a private nuisance action, is believed to be the first reported decision of a court awarding diminution in value damages, in addition to eradication costs, in a JKW case.

Background: Mr Williams and Mr Waistell ("the Claimants") owned two neighbouring semi-detached bungalows in Maesteg, South Wales, which were adjacent to a railway embankment belonging to Network Rail ("NR"). It was accepted at trial that JKW had been present on the embankment for in excess of 50 years. The Claimants had previously complained to NR, but treatment works carried out by NR were ultimately deemed inadequate.

Allegations: The Claimants brought alternative claims against NR: (i) that there was encroachment of JKW from land owned by NR, and, (ii) that the mere presence of JKW within close proximity of the boundary of the properties amounted to a serious interference with the use, quiet enjoyment and amenity value of their properties so as to equate to nuisance.

The Claimants initially sought an injunction requiring NR to treat the JKW (although this was not pursued with any vigour at trial), damages representing diminution in value, damages equating to the costs of treating the JKW themselves with an insurance backed guarantee, and general damages for distress / inconvenience.

Decision: It was accepted by all parties that a landowner has a duty in respect of all naturally occurring hazards or dangers to a neighbouring property and that this duty is measured in terms of reasonableness.

On this basis the Judge ruled that NR had sufficient knowledge (through the Claimants' complaints, internal notes about the JKW problem affecting embankments and the prior (unsuccessful) treatment of the offending JKW) to take all reasonable steps to prevent JKW from interfering with the Claimants' property. As NR had fallen short of taking all reasonable measures, breach of duty was established.

In relation to the encroachment issue, the Judge determined that whilst JKW rhizomes were present on the Claimants' land, there was no physical damage arising from the encroachment and, as such, a finding of private nuisance was not made out.

However, the Judge found that the presence of JKW on NR's land impacted upon the Claimants' ability to sell their properties at a proper value. The reason being that any potential lender would be unlikely to accept a lending proposition based upon security within close proximity to the JKW in issue (it being deemed as a Category IV risk).

The Judge further determined that even if treated and eradicated, and regardless of the fact that treatment would be backed by a guarantee with an insurance policy, there would be some stigma attaching to the Claimants' properties which, in the Judge's opinion, provided a basis for making a diminution in value award.

In addition to awarding each Claimant the costs of treatment with an insurance guarantee backed provider (£4,320), diminution in value damages were assessed at £10,000 per property.

Potential Consequences: This decision has serious ramifications for NR (tabloid headlines suggesting that claims totalling "tens of millions" could be brought), but also for all landowners, their buildings insurers and those valuing such property for secured lending purposes.

In 2012, the RICS issued its Japanese Knotweed Information Note. It is essential reading, particularly the pictorial guide to identifying this plant at all times throughout the year and the categories of risk defined by reference to the root systems' 'influencing distance'.

All surveyors should be familiar with what to look out for: fleshy red tinged shoots when it first breaks through the ground; large, heart or spade-shaped green leaves arranged in a zig-zag pattern along the stem; a hollow stem, like bamboo and clusters of cream flowers towards the end of July that attract bees. That is the easy bit.

The problem is that homeowners, alive to the difficulties that JKW will cause to sale prospects, often take steps to conceal any such outbreak which, even in the summer growing season, can render detection almost impossible within the site boundaries. Obvious locations beyond boundaries, i.e. railway embankments, river/streams, etc., should be considered but again this will often be constrained by physical barriers during the inspection.

Whether the Court of Appeal agrees with the trial Judge about residual diminution remains to be seen, and there is clearly a lot at stake should the decision be upheld. If that occurs, the plethora of claimant lawyers already advertising to attract homeowners who discover JKW will no doubt look to fill their boots keeping property and PI insurers, not to mention the relevant ombudsman services, extremely busy.