Japanese knotweed, an invasive plant species, remains a considerable worry for property owners across the United Kingdom. Its presence can not only cause structural damage to buildings but also significantly diminish a home's market value, making it harder to sell and potentially affecting mortgage applications. For years, the discovery of knotweed on or near a property has been a red flag for buyers, lenders, and surveyors alike, often leading to sales falling through or requiring extensive and costly eradication programmes.
The plant's aggressive growth habit sees it spread rapidly, with roots capable of penetrating foundations, drains, and paving. While its impact on physical structures has sometimes been overstated, the perception of risk alone has been enough to deter buyers. Consequently, properties affected by knotweed have historically seen their value reduced by 5% to 15%, and in some severe cases, even more, depending on the extent of the infestation and the proposed treatment plan. Homeowners are legally required to declare the presence of Japanese knotweed during the property sale process, typically via the TA6 Property Information Form.
Mortgage lenders have traditionally taken a cautious stance. Many lenders would refuse to offer finance on properties with knotweed within a certain distance (often seven metres) unless a professional treatment plan, backed by an insurance-backed guarantee, was in place. This often meant delays in the sales process and additional expenses for the seller. A typical professional treatment programme, which can take several growing seasons, might cost anywhere from £1,000 to £5,000, depending on the size of the infestation and the chosen method of eradication.
However, recent updates to guidance from the Royal Institution of Chartered Surveyors (RICS) aim to provide a more proportionate approach to assessing the risk posed by Japanese knotweed. The revised guidance, introduced in 2022, moves away from a rigid 'seven-metre rule' and instead encourages surveyors to make more nuanced, site-specific assessments. This shift acknowledges that not all knotweed infestations pose the same level of risk and that effective management solutions are available. The intention is to reduce unnecessary anxiety and allow more affected properties to proceed with sales, provided appropriate action is taken.
Despite the updated guidance, the onus remains on homeowners to manage and eradicate the plant. Failure to do so can lead to legal disputes if the knotweed spreads to neighbouring properties. While the RICS changes are a step towards a more pragmatic approach, the presence of Japanese knotweed will continue to be a material fact that must be disclosed and addressed. Property owners considering selling should proactively investigate and, if necessary, arrange for professional assessment and treatment to avoid potential complications and value depreciation.
The long-term implications for property values will depend on the effectiveness of treatment and the confidence of buyers and lenders in the revised assessment framework. While the stigma associated with Japanese knotweed may persist, a clear, documented history of professional treatment with guarantees can significantly mitigate its impact on a property's market appeal and value.