Can You Build On Land With Japanese knotweed?

By Paolo Martini on 4th February 2020 (updated: 17th February 2023) in Japanese Knotweed News | Knotweed Help

Japanese knotweed can cause financial difficulties to both homeowners and building developers, especially when plans have been made to build on land that is home to the invasive plant. Despite Japanese knotweed now being well-documented, homeowners and builders alike are still being presented with new scenarios which require legal guidance. Whilst there have been several precedents set in recent years that have solidified certain aspects of Japanese knotweed law, it’s not uncommon for landowners to be unsure of where they stand when it comes to building on land with Japanese knotweed.

Japanese Knotweed is not likely to cause significant property damage to buildings indirectly through subsidence or via collapse. However, Japanese Knotweed does cause structural damage to homes and properties via its extensive underground root system that pushes through city walls, drains and asphalt and above ground canopy that can exert persistent pressure on fences and flood defence systems. Because of this, Japanese Knotweed can cause significant delays and costs to building developments as many homeowners don’t know how to get rid of Japanese Knotweed themselves.

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Can new buildings be developed on land with Japanese knotweed?

New buildings can be developed on Japanese knotweed infested land, however, in order to legally do so, the presence of the plant should be declared and accounted for as part of the planning process. There have been several cases in recent years where building developers have chosen to ignore or hide the Japanese knotweed on their land, in order to avoid stalling their construction and to hasten the sale of the finished properties.

Unfortunately, building developers are not currently required to fill in the TA6 property form that is a standard part of the process in home exchanges, allowing them to lie about Japanese knotweed. This legal loophole has allowed new build developers to complete their work and then sell to homeowners who are unaware that they are buying a property affected by the invasive plant. Whilst there is no requirement for a TA6 property form, homebuyers can protect themselves by asking their conveyancing solicitor to ask the developer about any history of Japanese knotweed on the site.

Is a Japanese knotweed survey required before building development takes place?

A Japanese knotweed survey is not required on land before development takes place, however, if there’s a suspicion that the plant might affect the land then a survey could avoid time being wasted and potential money being lost. Japanese knotweed surveys can be undertaken by ecological experts or chartered surveyors and will either confirm or deny the presence of the plant on the land. In addition to identifying Japanese knotweed, a survey can help define the extent to which the plant affects the property, this could then impact any control plans that might be put together.

For example, if the Japanese knotweed is only discovered in an isolated patch, then it may be possible for this portion of land to be demarcated and for construction to continue regardless. Conversely, if the infestation is found to be widespread, then controlled excavations might need to be conducted in order to properly remove any remnants of the plant from the land before building takes place. Unfortunately, in some cases, surveyors miss Japanese knotweed, leading to a sale going through and a buyer being left out of pocket.

Is planning permission required to build on land with Japanese knotweed?

Planning permission is required before most kinds of building development takes place, this includes land with Japanese knotweed. Most councils will have policies in place concerning Japanese knotweed, and might also be aware of where the plant has already been discovered. Ignoring the presence of the plant and choosing to move soil contaminated with Japanese knotweed is a criminal offence under the Wildlife and Countryside Act 1981, which can lead to a heavy fine and even imprisonment.

In cases where the owner of the land is aware of Japanese knotweed on their own land, then they may need to make specific planning conditions alongside their application to show that they have factored the plant into their plans. Planning conditions set out how the Japanese knotweed will be controlled throughout the building development. This will include an assessment of control options, criteria for the completion of control measures and details on how those working on the site will prevent the further spread of the knotweed.

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Should Japanese knotweed be removed before building starts?

Japanese knotweed may need to be removed from land before building commences, depending on the severity of the infestation. This plant has the capability to grow up to 3 metres deep and 7 metres laterally from its visible point above ground, so in the case of large infestations, underground rhizome systems can sometimes be spread much further than may first appear. Due to the plant’s seasonal growth patterns, and tenacious growth characteristics, the entire removal of the Japanese knotweed could be required in order to ensure that it does not disturb foundations, or exploit any weaknesses whilst searching for a route to moisture and sunlight.

There a number of laws that control the removal of Japanese knotweed contaminated waste, additionally, building contractors are subject to more regulations that private landowners. In order to avoid large fines, developers should ensure that only registered waste carriers are used to dispose of any contaminated soil. It’s illegal to dispose of Japanese knotweed along with other soil, and it’s also recommended to inform the chosen waste disposal site before dropping off the controlled waste.

Will Japanese knotweed grow through new builds?

Japanese knotweed has been known to grow through new builds, however, this usually only occurs in circumstances where an infestation is particularly large. Although the destructive ability of Japanese knotweed has been exaggerated by some media sources, the plant still remains a force to be reckoned with. In a recent report by the Science and Technology Committee, experts from across the industry gave their findings on how the plant affects buildings, and whilst many agreed that it was capable of damaging buildings, it was admitted that this damage was no worse than what could be done by common trees.

With that being said, it was noted that whilst trees such as buddleias could feasibly do more damage to property than Japanese knotweed, these trees were much easier to remove. It was agreed that whilst the destructive abilities of Japanese knotweed had been overblown by some in the industry, their hardiness and resistance to traditional forms of removal had not been understated.

How much could property be devalued, if it’s built on Japanese knotweed?

A property can be devalued by as much as 20%, if Japanese knotweed is discovered on the land it is built on. In extreme cases, a home can be almost completely devalued by Japanese knotweed, such as in the case of the Jones’, a family in Bedford who were told that their new build had dropped in price from £350,000 to £50,000, after having lived there for one month. It should be noted that, besides the devaluation of the property, there might also be additional costs incurred as a result of building on land with Japanese knotweed.

For example, if the seller attempts to lie about the presence of Japanese knotweed on their land, then they could be sued for misrepresentation by the buyer. In a survey of 100 individuals who had been affected by Japanese knotweed (undertaken by the Crop Protection Association), 15% had seen a property deal through as a result of the discovery of the plant, whereas 20% saw a drop in their house value and 10% were forced to pay compensation of some kind as a result of finding the plant on their land.

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Do you have to declare Japanese knotweed after it’s been built on?

Property owners do not have to declare Japanese knotweed after building on the land if they are planning on staying on the property, however, they should make any potential buyer of the property aware about the presence of Japanese knotweed, otherwise, they may be liable for misrepresentation. Any person choosing to live on land affected by Japanese knotweed will be legally responsible for ensuring that it does not spread to neighbouring properties. If the plant is allowed to spread, whether during the construction process or by any other means, the owner of the land will be liable to fines or imprisonment.

Is it possible to build on land with Japanese knotweed while it’s being treated?

It is possible to build on land with Japanese knotweed while it is being treated, depending on the size of the infestation, the choice of treatment and the planned development. Unfortunately, in cases where there is a concentrated infestation over a small area of land, it may be impossible to undergo construction until the Japanese knotweed has been dealt with. A thorough Japanese knotweed survey, in association with a set of planning conditions, can help to decide whether the development will be able to begin before, after or during the treatment of the plant.

When a Japanese knotweed infestation of 4 acres was discovered before work was due to be undertaken on the Olympic Park for the 2012 games, it was decided that treatment and removal of the plant could be managed in tandem with construction. Despite the 4 hectares comprising less than 2% of the entire park, the overall treatment ended up costing £70 million and took years to complete. Due to the strict limitations placed on the movement of knotweed contaminated waste, the Environment Agency, London Development Agency and Olympic Delivery Authority devised a varied approach including glyphosate treatment, burning and burial beneath a protective membrane.

Will Japanese knotweed deter developers from buying land to build on?

The presence of Japanese knotweed will likely act as a deterrent for most developers who are planning on building on the land. Whilst Japanese knotweed is not treated with the same level of outright fear as it once was, its presence devalues property and can cause delays in the sale of the land. In order to build on land that has Japanese knotweed, developers should set out planned conditions, so that they do not inadvertently spread the plant, and they should also inform any future buyers that Japanese knotweed is present, regardless of if it has already been treated.

If you have discovered Japanese knotweed on your land and are unsure if you should build on it, or if you’ve found it on a property that you’ve recently bought, then we may be able to offer you some legal guidance. Contact us using the contact form, or give us a call on 0151 668 0554 to find out if we can help. 

Paolo martini cobleys solicitor
Paolo Martini

Paolo Martini is the lead solicitor for Knotweed Help and has over 30 years of experience in the field of Civil Litigation and is an expert on the legal issues faced by individuals dealing with Japanese knotweed. Now considered one of the country's leading litigators in Japanese knotweed law he works alongside the country’s top barristers and experts. His in-depth legal experience and connections to the Japanese knotweed removal industry make him uniquely suited for handling your case.