RICS Draft Guidance Note: Japanese knotweed and residential property, 1st edition

1 Introduction

1.1 Scope

This guidance note replaces the RICS information paper Japanese Knotweed and residential property, 2012. It reflects an improved understanding of Japanese knotweed and, like its predecessor, its primary objective is to assist RICS members who encounter Japanese knotweed while undertaking valuations or surveys of residential property in the UK. It is intended to help residential practitioners make a preliminary assessment and to provide appropriate initial advice to their clients, whether inspecting for mortgage lending or pre-purchase purposes.

This introductory section outlines developments in the understanding of Japanese knotweed since the original information paper was published in 2012 and references more recent publications, which have influenced its development.

Sections 2 and 3 of the guidance note summarise the nature of the Japanese knotweed problem and briefly consider aspects of the RICS regulatory context within which RICS members undertake surveys and valuations. These will be largely familiar to RICS members but the introduction of the RICS Home Survey Standard, which became effective on 1 March 2021, may require some adjustments to established practice. The wider background perspective may assist the understanding of non-members.

Section 4 sets out a revised assessment methodology to help valuers and surveyors objectively describe the scale of a Japanese knotweed infestation. This will help other stakeholders to make balanced and measured decisions. Sections 5 and 6 deal with reporting to and advising clients, while Section 7 outlines factors that may need to be considered when valuing a residential property affected by Japanese knotweed.

This guidance note provides a framework for RICS members to categorise infestations of Japanese knotweed and only briefly summarises the main methods of remediation. Further information about the identification and remediation of Japanese knotweed can be found in the publication by the Property Care Association Japanese knotweed: Guidance for Professional Valuers and Surveyors, which deals with these important subjects in depth.

This guidance note should be read in conjunction with the latest edition of the RICS Valuation - Global Standards (Red Book Global Standards). However, its scope extends beyond valuations conducted in accordance with the Red Book Global Standards. Residential practitioners are, therefore, also encouraged to consider this guidance note when carrying out the following professional services:

  • preparing valuations for the purposes of agency
  • providing level one, level two and level three surveys
  • Single Surveys in Scotland.

The guidance note includes reference to the main UK legislative and regulatory provisions that affect, or are considered likely to affect, the value of residential properties where Japanese knotweed is present. Although these legislative and regulatory frameworks differ between England, Wales, Scotland, and Northern Ireland, the essential features governing Japanese knotweed control and disposal are broadly similar.

1.2 Background

Japanese knotweed is a hardy bamboo-like perennial plant that grows quickly and strongly. It spreads through its underground rhizomes or shoots and thick clumps or stands can quickly grow to a height of over 2m during the spring and summer. Introduced into the UK in the mid-19th century it was initially popular with landscapers because of its ability to grow quickly and form dense screens.

However, it soon became a problem due to its propensity to spread and establish easily. Its biological and physiological characteristics have allowed it to take advantage of human interference, ground disturbance and the movement of 'contaminated' soil. Its ability to out-compete indigenous plant life, lack of natural predators and ignorance on the part of landowners has resulted in its widespread distribution to all parts of the UK, Europe and North America.

Japanese knotweed was included in the original list of invasive alien species (so-called 'Schedule 9') in the Wildlife and Countryside Act 1981. This made it an offence to 'plant or otherwise cause Japanese knotweed to grow in the wild'. This recognition of environmental impact is why soil or plant material contaminated with Japanese knotweed is likely to be classified as 'controlled waste' under the Environmental Protection Act 1990.

Japanese knotweed caused increasing problems in the residential market due to concerns that the plant may cause damage to properties. These concerns originated from an understanding that left unmanaged, Japanese knotweed can rapidly colonise and dominate not just green areas but can affect hard landscaped areas and can be seen disrupting lightweight structures and masonry garden walls.

1.3 Previous research and publications

The RICS information paper published in 2012 established a protocol for the objective assessment of the risk to a property posed by Japanese knotweed. It specified four risk categories, using a distance of 7m from buildings and boundaries as the defining measurement. This 7m distance was recognised at the time as being potentially conservative but it had previously been used in an Environment Agency publication and no justifiable alternative could be identified.

Practical experience after 2012 increasingly questioned a widely held assumption that Japanese knotweed posed a structural risk to building foundations. During 2016, practicing RICS surveyors and specialist contractors contributed data from surveys towards research on the typical impact of Japanese knotweed. The research paper by Fennell et al, (Japanese knotweed (Fallopia japonica): An analysis of capacity to cause structural damage (compared to other plants) and typical rhizome extension) published in 2018, reported that Japanese knotweed poses less of a risk of damage to properties than many trees or woody shrubs. It also led to a general agreement that 3m is a more appropriate distance measurement of typical spread of the root/rhizome network in the soil than the 7m distance adopted for the original risk assessment.

Meanwhile, an extended research project on alternative remediation methods in South Wales, also published in 2018 (Optimising physiochemical control of invasive Japanese knotweed), identified the optimum method of controlling and remediating Japanese knotweed. Although this research acknowledged the difficulty of eradicating well-established stands of Japanese knotweed within a short timescale it confirmed that control of an infestation was readily achievable when properly undertaken.

In 2019, the House of Commons Science and Technology Committee published a report on an enquiry titled Japanese knotweed and the built environment. This report acknowledged the role of the existing RICS risk assessment framework in providing lenders with the confidence to lend against properties affected by Japanese knotweed. However, it described the '7m rule' as a blunt instrument that did not reflect the latest scientific evidence and called for a revised assessment process which is 'much more nuanced and evidence-based ... to reflect the latest thinking on the significance of Japanese knotweed, in relation to the size of the infestation, the distance from the property, and the potential risk of any damage'.

In 2020, in response to the House of Commons publication, Risk & Policy Analysts (RPA) published a report on behalf of the Department of Environment, Food and Rural Affairs (Defra) on an investigation into Japanese knotweed, comparing the approach taken in the UK with other countries (International Approaches to Japanese Knotweed in the Context of Property Sales). The research found that 'no other country takes a similar approach to Japanese knotweed in the context of property sales as the UK', while concluding that 'the UK approach is not disproportionate given the level of invasion of knotweed and the control measures that may be required ... '. However, it noted that within the UK housing market, 'excessive burden stems from the stigma surrounding Japanese knotweed in the UK, where beliefs about its effects are exaggerated, creating nervousness among buyers, sellers and lenders during the property buying process. This impacts the efficiency of the approach as parties become unduly risk-averse'. It also commented that 'attitudes are currently disproportionate to the physical risk posed by Japanese knotweed'. As a direct consequence, 'the media, and as a result the public, have a disproportionate fear of the problem'.

The Defra report made recommendations for changes in approach within the UK, including:

  • reassessment of the risk categories presented in the RICS guidance to provide clarity for lenders and enhance coherence in lending policy
  • provision of consistent professional advice to reframe Japanese knotweed as a mitigatable environmental issue, rather than solely as a property or social issue
  • recognition of an increasing awareness that eradication of Japanese knotweed is not a helpful objective, and that the focus should be on management and control
  • public education and awareness raising campaigns are needed to address the stigma of knotweed and reassure potential house buyers/sellers that the implementation of management plans for Japanese knotweed provides appropriate mitigation. Such a campaign would help foster the necessary reassurance to both lenders and buyers to enable balanced and measured decisions to be taken in the context of property sales
  • a change in language by professionals to enable the 'normalisation' of knotweed as an environmental issue rather than only being a social/property issue. This would still enable Japanese knotweed to be managed as an invasive species, but fully considering its environmental impacts rather than only its physical impacts on property.

This guidance note directly addresses the concerns of the House of Commons Science and Technology Committee and responds to the issues raised in the report by RPA on behalf of Defra by introducing a new assessment process. The professional understanding of Japanese knotweed has increased significantly since 2012 and evidence from the latest research has enabled the assessment process to be focused on the need to manage Japanese knotweed rather than any perceived risk to the structure of dwellings. The distance-based 'blunt instrument' of the previous assessment process has been replaced with a more specific consideration of the impact of an infestation at a property. However, the new process still delivers a straightforward and objective categorisation of Japanese knotweed infestations because this provides the clarity that is essential for lenders and which will continue to ensure confidence and trust among the wider stakeholders.

As the 2012 RICS information paper acknowledged, the residential property market has faced controversial issues before, which have been largely resolved and assimilated into valuation and survey processes. This guidance note is the next step in managing the adjustment within the residential property market to the issues posed by Japanese knotweed.

1.4 Effective date

This guidance note is effective three months from publication.