Japanese knotweed now affects between four and five per cent of homes in the UK, according to Environet UK.

It estimates that around 850,000 to 900,000 UK households are affected by the invasive plant and will suffer an average reduction in value of 10%, knocking almost £20 billion (£19.8 billion) off UK property values.

The rapid spread of Japanese knotweed across the country and growing awareness of the risks means homeowners are increasingly forced to confront the issue when buying and selling property.

Caution from lenders

Mortgage lenders are highly cautious about lending against properties affected by the plant, which can make the properties more difficult to sell.

To help homeowners understand how Japanese knotweed can affect the value of their property Environet and Expert Surveyors Ltd have developed an algorithm.

The online tool – called JK-VIM – enables buyers and sellers to assess how much the value of a home is reduced if it is affected by knotweed and offers revised valuations if the infestation is treated by herbicide or excavation and has a 10-year insurance backed guarantee for the works.

It means that sellers are better able to work out if it’s financially worthwhile to have the knotweed professionally treated before selling the property, whilst buyers are in a stronger position to negotiate a lower price that is both accurate and fair.

Nic Seal, MD of Environet, said: “Lending practices for properties with Japanese knotweed are in urgent need of review, considering all that’s usually required is a professional treatment plan to deal with the problem and a sensible readjustment of the price. In reality, properties with knotweed are worth on average around 10% less than an unaffected property, which reflects the cost of treatment and the so-called ‘knotweed stigma’.

“Now, for the first time, sellers will be able to see for themselves whether it’s worth their while financially to have an infestation professionally treated before marketing a property, by seeing exactly how much its value is diminished. Buyers, meanwhile, can get a realistic idea of how much they should be offering.”