Plants wrongly destroying value of thousands of homes

Japanese knotweed (Fallopia japonica), a fast-growing invasive plant with bamboo-like stems, is causing many homes to become near impossible to sell, even when it poses no real threat. An investigation and report by the House of Commons Science and Technology Committee has found that banks and mortgage lenders are relying on discredited scientific evidence and devaluing homes if the plant is growing nearby.

The report criticises the continued use of the “seven-metre” proximity rule, a rule proposed in 2012 by the Royal Institution of Chartered Surveyors (RICS). However, this rule has since been abandoned due to not having a strong enough evidence base.

Japenese knotweed was first introduced to the UK in the nineteenth century in gardens and parks. It has since become a pest however, as it can grow through and crack tarmac and paving slabs. It can also grow through existing cracks in buildings, which then makes them worse.

The plant is also very difficult to remove, as it requires either complete excavation or years of treatment with chemicals, and this can cost around £5,000.

While the report agrees that Japanese knotweed can cause damage, it has brought attention to recent evidence which proposes that the effect on buildings “might not be as significant as previously believed.'' The report also pointed out that other invasive plants do not have the same result on property prices, and that lenders are being “overly cautious” when it comes to this one.

A 2018 study by the University of Leeds with engineering firm Aecom found that that while Japanese knotweed can worsen existing cracks in structures, the plant prefers to grow around obstacles rather than through them. They also argue that the plant does not cause substantial damage to buildings, even when it grows close by. The research examined 68 homes where knotweed was found, as well as 81 additional sites.

The House of Commons Science and Technology Committee has also spotted a difference between UK lenders and their European counterparts; mortgage companies on mainland Europe are far less cautious.

Norman Lamb MP, Chair of the Science and Technology Committee, said, “It is clear that the UK’s current approach to Japanese knotweed is more cautious than it needs to be, especially when comparing it to that of other countries. We need an evidence-based and nuanced approach to the issue, one that reassures owners and buyers that they will not be subject to disproportionate caution when trying to sell or buy a property.”

It has been estimated that in Britain more than 1.25 per cent of residential properties, and over two per cent of development sites, are affected by the plant. This comes to tens of thousands of sites in total. A 2017 survey by the Crop Protection Association suggested that, among properties affected by knotweed, 15 per cent had their sale fall through, 20 per cent saw a drop in the property’s value, and 10 per cent had to pay out compensation due to the plant.

The report recommended that the Department for Environment, Food and Rural Affairs, commission a study of international approaches to Japanese knotweed’s result on property sales.