The Court of Appeal has found that landowners may be liable to pay damages if Japanese knotweed (including the plant, or its roots or rhizomes) encroaches from their property onto neighbouring land.

In Network Rail v Williams and Waistell, the Court of Appeal found that:

• Knotweed encroachment constituted “an unreasonable interference” with the enjoyment of the claimants’ properties. The extensiveness of knotweed roots and its rhizomes has the potential to prevent development and restricts a property’s amenity value;

• The encroachment of knotweed onto neighbouring land had caused a loss of amenity and risk of future physical damage. This was an actionable basis for a nuisance claim, for which in an appropriate case a claimant could obtain an injunction and/or damages; and

• A final mandatory injunction should be available to a claimant even though there has not been any physical damage to the property caused by knotweed.

Developers should have regard to this decision when purchasing sites. In the past developers' main concerns about Japanese knotweed have related to costs of treatment, potential delay while treatment takes place, damage and the ability of plot purchasers to obtain mortgages on previously infested sites. This case crystallises the real potential for further risk of third party land owner claims and emphasises the need for developers to address knotweed infestation at the earliest possible stage.

If Japanese knotweed is identified on site prior to exchange of contracts developers should carry out a thorough review of environmental indemnity provisions to ensure that they do not inadvertently inherit liability for claims relating to historic encroachment.