Around 4-5% of houses are currently affected directly or indirectly by knotweed, and approximately 850,000 to 900,000 UK households are suffering an average reduction in value of around 10%, according to research by the Japanese knotweed removal firm Environet UK.
Mortgage lenders will typically refuse loans against affected properties unless there is a professional knotweed management plan in place with an insurance-backed guarantee.
Sellers are also required by law to inform potential purchasers whether a property is, or has been, affected by Japanese knotweed, which can act as a deterrent, even if the infestation has been treated.
According to research conducted by the University of Leeds and infrastructure services firm AECOM, there is no evidence Japanese knotweed caused significant structural damage.
Some academics believe refusing mortgages on properties where Japanese knotweed has been found is out of proportion to the risk posed by the species.
As a result, mortgage lenders have been urged to reassess their lending policies taking into account the new data.
Impact on values by deterring buyers and making homes difficult to sell
Founder and managing director of Environet, Nic Seal, said that Japanese knotweed is having a serious impact on values by deterring buyers and making homes difficult to sell, even if the knotweed has been successfully treated.
He added: “Several high profile legal cases this year, whereby landowners were successfully sued for allowing Japanese knotweed to encroach onto neighboring properties, have led to greater awareness among the general public of the damage Japanese knotweed can cause to homes and house prices.
“Those affected by knotweed should seek to resolve the issue as quickly as possible by appointing a professional removal firm and securing a ten-year insurance-backed guarantee for the work, which will can be passed onto a buyer and their mortgage lender.
“DIY attempts at treatment will usually only make things worse and can even hasten the spread of the plant.”