The impact of Japanese Knotweed on home ownership and property values

Date: February 2024
Author: Paul Raine BSc FRICS MEWI
Company: Expert Surveyors Ltd
Author's expertise: Paul Raine is one of the leading RICS valuation experts in the UK on matters relating to the impact of Japanese Knotweed on residential property values. Paul regularly speaks at conferences and seminars, Chairs the UK Knotweed Valuation Forum and provides expert witness reports on knotweed to a wide range of legal and corporate clients. Paul has given evidence in court on knotweed issues relating to claims of diminution in value associated with knotweed.

This blog is intended to provide homeowners with clear advice and answers to questions if they find themselves affected by an infestation of Japanese Knotweed, either on their land or on neighbouring land within close proximity to their house.

Q1: Is Japanese Knotweed harmful to people or animals?

A1: No. Japanese Knotweed is not harmful to people or animals.

Q2: Does Japanese Knotweed damage buildings and garden walls?

A2: Very rarely and only normally if the buildings and garden walls are already in poor condition. The ‘hype’ about Japanese Knotweed causing damage to buildings is mostly incorrect.

Q3: Does Japanese Knotweed damage paths, patios and driveways.

A3: Yes, it can do but again usually only if they are already in worn condition. It certainly does grow through cracks and joints, as any weed can do. It is just that Japanese Knotweed is an incredibly invasive weed.

Q4: Does Japanese Knotweed reduce the value of my home?

A4: Yes, in most cases it will reduce the value of your home for the period of the treatment and for several years after. Each case is different and depends on a combination of 10 key factors. Contact amanda@expert-surveyors.co.uk if you require a professional valuation report. There is a real stigma associated with Japanese Knotweed and most willing buyers would much rather buy a house without an infestation of Japanese Knotweed (or history of an infestation of Japanese Knotweed). Therefore, it follows that in most cases willing buyers will only pay below market value if a property is infested with Japanese Knotweed.

Q5: Can I still get a mortgage if my home has Japanese Knotweed?

A5: In nearly all cases, only if you put in place a professional Knotweed Management Plan provided by an approved contractor – i.e. a contractor who is a member of the Property Care Association and who can provide a 10-year insurance-backed guarantee on completion of the treatment. See https://www.property-care.org/skill/invasive-weed-control/

Q6: What is a professional Knotweed Management Plan?

A6: This is a treatment plan put into effect by a professional knotweed contractor who can provide a 10-year insurance-backed guarantee on completion of the treatment. It can be based on herbicide-treatment or excavation, or both. We can provide you with further details of PCA approved contractors.

Q7: What is the difference between herbicide treatment and excavation?

A7: The main difference is cost – excavation can be many times more expensive than herbicide treatment and which method is the most cost-effective will vary on a case by case basis. There is definitely not a preferred method of treatment, although instinctively most homeowners will think excavation must be best. It requires contractor expertise and property valuation expertise to identify which is the best method of treatment for the particulars of the property and the infestation.

Q8: What do I do if I want to build on the land which has Japanese Knotweed?

Q8: Put simply, you will need to excavate and remove the infested soil using a PCA approved contractor. You must take professional advice before doing anything and for development land, the impact on market value can result in a substantial diminution in market value, due to the cost of excavation and the risk of building on land which has been remediated.

Q9: I have heard you can never get rid of Japanese Knotweed once a garden or land is infested. Is this correct?

A9: It is the case that Japanese Knotweed can be notoriously difficult to eradicate and you will note that the Knotweed Plans are called management plans, not eradication plans. The emphasis is on control, not eradication and that it one of the reasons why Japanese Knotweed causes diminution in value.

Q10: What are my legal obligations if I have Japanese Knotweed on my land?

A10: The responsibility of managing Japanese knotweed lies with the owner or occupier of the site where knotweed is present. The legislation in place covering the control and disposal of Japanese knotweed comprises:
The Wildlife and Countryside Act 1981
Section 14(2) of The Wildlife and Countryside Act 1981 (WCA 1981) states that 'if any person plants or otherwise causes to grow in the wild any plant which is included in Part II of Schedule 9, he shall be guilty of an offence. Anyone convicted of an offence under Section 14 of the WCA 1981 may face a fine and/or 6 months imprisonment. Japanese knotweed is listed in part II of the schedule.’
The Environmental Protection Act 1990
'Any Japanese knotweed contaminated soil or plant material that you intend to dispose of is likely to be classified as 'controlled waste' under The Environmental Protection Act 1990. Untreated knotweed is not regarded as a 'hazardous waste' under the Hazardous Waste Regulations 2005 but material containing knotweed that has been treated with certain herbicides could be.’

Q11: What are my neighbours’ legal obligations if they have Japanese Knotweed on their land which is within 7 metres of the boundary of my property?

A11: Put simply, they need to put in place a professional Knotweed Management Plan. If they don’t, they could potentially cause your property to become unsuitable mortgage security and in most cases, that would have a major adverse impact on value and a substantial diminution in value.

Q12: I have heard that the underground roots (rhizomes) can extend 7 metres underground in a radius around the visible above ground aerial infestation. Is that correct?

A12: The rhizomes can grow to depths of two meters and rapidly extend up to seven meters horizontally from the visible part of the plant and the stems can grow up to 7ft high.

Q13: If I find Japanese Knotweed, do I need to tell my Buildings insurers?

A13: You need to check your household insurance policy.

Q14: Is there an additional complication if my Japanese Knotweed infestation is near a watercourse?

A14: Yes. The contractor will need to obtain an Environment Agency Licence to apply herbicide within 5 – 10 metres of a watercourse.

Q15: Have there been any research papers on Japanese Knotweed?

A15: Yes. In 2018, there were academic research papers on Japanese Knotweed produced by Leeds University and Swansea University which help inform the discussion on the treatment of Japanese Knotweed and its impact on market value.

Q16: If I need to make a claim for compensation for the loss in value of my property, are there any specialist lawyers who can help me?

A16: Yes. We work with several law firms who specialise in Japanese Knotweed claims.

Q17: Can I claim against the seller if I buy a property and they do not tell me about Japanese Knotweed on the land when they sold it?

A17: Yes. In most cases a claim for negligent or fraudulent misrepresentation can be made if a seller fails to disclose Japanese Knotweed on their land.

Q18: Can I claim against my surveyor if I have a survey done prior to buying the property which fails to identify and report Japanese Knotweed on my property?

A18: Yes – assuming the Japanese Knotweed would have been visible to the surveyor and there are guidance notes and legal decisions on surveyor’s liability. We can provide further advice.

Q19: I have heard there is a “10% rule” – that a property affected by Japanese Knotweed is worth 10% less than the same house without it. Is that correct?

A19. No – that is not correct and is based on the incorrect application of one court judgement. There is no such “rule” and diminution in value needs to be assessed on a case by case basis.

Q20: If I need to appoint a PCA approved contractor to treat my Japanese Knotweed, do you know who are the most experienced contractors?

A20: Yes, we work with several of the industry-leading PCA approved contractors and will be pleased to refer you.

Q21: Are there RICS valuation experts who can provide reports on diminution in value as a result of an infestation of Japanese Knotweed?

A21: That is what we do. We have a national network of RICS valuation expertise with specialist training in the assessment of diminution in value associated with Japanese Knotweed.

Q22: Do I need to disclose any issues of Japanese Knotweed when I come to sell my property?

A22: Yes – definitely. If you do not you will be liable to receive a claim from the new purchaser. It is a legal requirement to disclose and there is a specific question (7.8) relating to Japanese Knotweed on the Law Society Property Information Form TA6.

Q23: Is Japanese Knotweed all a ‘fuss about nothing’ and the product of media hype that will go away when the media have lost interest?

A23. There is no current evidence that the negative impact of Japanese Knotweed on home ownership and property values is going to recede or go away any time soon. There is a real stigma associated with Japanese Knotweed and most willing buyers would much rather buy a house without an infestation of Japanese Knotweed (or history of an infestation of Japanese Knotweed).