Japanese knotweed on a neighbour's land

If Japanese knotweed spreads onto your land from next door, you may have a claim in private nuisance, and in England and Wales you need not wait for it to cause physical damage. A fall in value is not enough on its own, and since the Supreme Court's 2024 decision in Davies v Bridgend you must also show that the neighbour's breach caused the loss you are claiming for.

Both leading cases are England and Wales law, and every dispute turns on its own facts.

Network Rail v Williams

The Court of Appeal decided Network Rail Infrastructure Ltd v Williams and Waistell on 3 July 2018. Knotweed had grown on a Network Rail embankment behind two bungalows in Maesteg, South Wales, for at least 50 years.

The judgment makes three main points. First, a fall in market value caused only by knotweed on neighbouring land is not, on its own, actionable, because nuisance protects the use and enjoyment of land rather than its value as an investment.

Second, knotweed rhizomes spreading onto a neighbour's land are an actionable nuisance even before they cause physical damage. The court described knotweed as a "natural hazard" that places an immediate burden on the owner and reduces their ability to use and enjoy the land.

Third, a landowner who knows, or ought to know, of the risk and fails to take reasonable steps is liable. The court said Network Rail ought to have known by 2012, when the Environment Agency's code of practice and the RICS paper were published.

The appeal was dismissed, although the original decision was upheld for different reasons. The awards were:

The Court of Appeal did not revisit the residual diminution figures on their merits. It added that, in an appropriate case, a court could grant a final mandatory injunction where knotweed roots reduce the amenity value of land, even before any physical damage.

Davies v Bridgend

In Davies v Bridgend County Borough Council, decided on 8 May 2024, the Supreme Court unanimously allowed the council's appeal. Knotweed had spread from council land onto Mr Davies's land well before he bought it in 2004. The council was in breach only from 2013, when it knew or ought to have known of the risk, until 2018, when it began an effective treatment programme.

The Supreme Court held that Mr Davies could not recover the £4,900 for residual diminution in value that the Court of Appeal had awarded. Applying the "but for" test, the fall in value had happened long before the breach began, so the breach did not cause it. He had already conceded at trial that his £3,600 claim for treatment costs could not be recovered, because he would always have had to treat the knotweed.

Williams remains the leading Court of Appeal authority on knotweed encroachment as a nuisance. In its full judgment, the Supreme Court also declined to consider the effect of the 2022 RICS guidance, which replaced the 2012 paper.

In practice, a claim depends on when the neighbour knew, or ought to have known, of the risk. If the knotweed had already spread and your property had already lost value before the neighbour was in breach, a claim for that loss is likely to fail. Telling your neighbour in writing gives you a dated record of when they knew.

Practical steps

1. Confirm that it is knotweed. Our photo check gives an automated, indicative answer, but for anything you may rely on later, RICS says investigation should be by an accredited member of a recognised trade body. In 2022 it named the Property Care Association (PCA) and INNSA.
2. Gather evidence: dated photographs from your side of the boundary, the distance from the boundary to the nearest stems, any shoots on your land, and copies of every message.
3. Get a written knotweed survey from a specialist, so that you and your neighbour are working from the same facts.
4. Talk to your neighbour and share what you have found, then follow up in writing, asking what they plan to do and by when. In England, government guidance says owners must stop knotweed on their land from spreading off their property.
5. If talks stall, a mediator can help you agree a joint plan, such as treating both sides of the boundary at the same time.
6. Check whether your home insurance includes legal expenses cover, and speak to a solicitor before you issue a claim.
7. Report persistent problems to the council.

Community protection notices

Home Office guidance says a community protection notice (CPN) can require someone to control or prevent the growth of Japanese knotweed. CPNs apply in England and Wales, and as RICS notes, they are not directly available to private homeowners: only a council or the police can issue one, and usually it will be the council.

The test is conduct, including a failure to act, that is unreasonable and has a persistent or continuing detrimental effect on the quality of life of those in the locality. A written warning must come first. The Home Office note adds that the community trigger, a formal review of an anti-social behaviour case, could apply to persistent knotweed problems, and a person who fails to comply with a notice can face penalties.

Sales and mortgages

Under the RICS professional standard, knotweed that is not seen on your property but is visible on adjoining land within 3 metres of your boundary falls into Management Category D. RICS describes the impact as limited: no action or mortgage retention should be needed for lending, except in exceptional circumstances. Lenders should not require you to treat a neighbour's knotweed as a condition of a mortgage, RICS adds, because you cannot control it.

Knotweed along the boundary, in boundary hedging or affecting boundary walls counts as on site, so category A, B or C applies instead. Knotweed more than 3 metres beyond the boundary is normally only recorded in the valuer's notes, although a large, mature, unmanaged stand may exceptionally still be reported as Category D.

The effect on value is usually limited unless a severe infestation next door makes expensive defensive measures worth considering, and RICS notes that root barriers may not be viable or effective for many homes. RICS presents its framework as guidance rather than a definitive rule, and each lender sets its own policy.

If you sell your home, the Law Society's TA6 explanatory notes say a property is usually considered at risk if knotweed grows on it or within three metres of its boundary. Buyers can rely on what you say, you must say if you do not know an answer, and you should attach any survey report or management plan.

Council and railway land

Both leading cases were brought against public landowners: Williams concerned a Network Rail embankment and Davies council land. In each, the owner's breach was dated from when it knew, or ought to have known, of the risk.

For valuations, RICS gives statutory-authority land, land let by an absentee landlord and derelict sites as examples where a widespread, mature and unmanaged stand more than 3 metres from the boundary may still be reported as Category D.

The practical steps are the same as with a private neighbour: gather evidence, get a survey and write to the landowner, keeping a copy of everything you send.