Japanese knotweed and its impact on house prices

Japanese knotweed can derail a sale, reduce an offer and complicate a mortgage. Here's what buyers and sellers need to know before they proceed.

There are many things that can have an impact on your plans to buy a property, with finances almost certainly being at the top of the list.

The ongoing cost-of-living issues have been exacerbated by the impact of the wars in Ukraine and in the Persian Gulf applying pressure on fuel costs which then spreads to high street prices. As the average pocket is put under strain, prospective housebuyers are becoming less likely to want to spend a lot of money on buying their next home, which has had a depressive effect on the housing market.

The housing market has - for the time being, at least - become a buyers' market with sellers being forced to reduce their prices to attract buyers to purchase their property.

But, other than market forces, there are additional factors affecting what buyers are prepared to purchase and what that means for agreeing a price.

Most people are probably aware of some of the common issues that might afflict a property, such as damp, subsidence and roof problems, which can result in expensive bills to put right. This is why a survey is recommended to establish the condition and stability of the home before going ahead with its purchase.

Japanese knotweed is a non-native invasive plant that was brought into the UK by avid Victorian gardeners and, from there, it has proliferated throughout the country. It is of particular concern because of its rapid growth and propensity for causing structural instability in homes. The plant has rhizomes that spread underground, with its shoots finding their way into areas of the property that are weak — for example, cracks in walls or hard-standing — and expanding with rapid growth that pushes the structure apart.

A detailed survey will pinpoint evidence of its growth around the grounds of the property, as well as identifying its existence on neighbouring properties where access allows. The chartered surveyor may then recommend that an expert be brought in to assess the issue and quote for any remedial work and the plant's removal. For a full breakdown of what to expect, Surveyor Local's guide to Japanese knotweed surveys covers the process in detail.

Because of its impact on property, it is now a legal requirement for sellers to declare their knowledge of its existence on the conveyancing forms, which are passed on to the buyer via the solicitors. While it is not illegal to have Japanese knotweed on the property, it is illegal not to have declared knowledge of its existence in the selling particulars. Because of this, there is a stigma attached to properties where its existence has been declared and, because of the associated removal costs, potential buyers may be put off putting in an offer.

Such an expert is Environet, which notes that costs of treatment can range from £2,000 to over £20,000 in some cases, the exact amount being dependent on the extent of the problem, the size of the issue and the chosen method of removal.

In addition, many mortgage lenders require a formal treatment plan to be submitted before they are prepared to make an offer to a borrower, usually with an insurance-backed guarantee from the expert removers.

This reticence to buy a property with Japanese knotweed means there is a downward pressure on what is an acceptable price to offer. If the buyer is not completely put off, Environet estimates that there could be a 10% to 15% price reduction. Even the knowledge of a previous infestation can affect the offer made, which is why it is important to obtain a formal treatment plan from an expert, along with the insurance-backed guarantee, as the buyer will be protected.

Environet also provides a heat map detailing the extent of the proliferation of Japanese knotweed throughout the country. You can search for the location where you are looking to buy a property and understand the likelihood of the invasive plant being present.

Such problems are unfortunately part and parcel of the home-buying and -selling process, which is why it is important to have a focused and experienced conveyancing solicitor working on your transaction. If you have questions or need clarity on any of the issues affecting a sale or purchase, a Homeward Legal solicitor can guide you through it.

Homeward Legal operates a no completion, no fee promise, and will provide a fixed quote so you can budget with confidence. Call our friendly team on or use our quick quote tool to get started.