Remortgage With Japanese Knotweed

What Is Japanese Knotweed and Why Does It Matter?

Japanese knotweed (Fallopia japonica) was introduced to the UK in the mid-nineteenth century as an ornamental garden plant. It has since become one of the most invasive non-native species in the country, capable of growing at a remarkable rate of up to 20 centimetres per day during the growing season and reaching heights of three metres or more.

Why it concerns lenders

Lenders worry about Japanese knotweed for several interconnected reasons. The plant can cause physical damage to buildings, paths, driveways, drainage systems, and boundary walls. Its root system (technically a rhizome network) can extend several metres from the visible plant and penetrate through small cracks in concrete, tarmac, and masonry. This potential for structural damage directly threatens the value of the property that serves as the lender's security.

Impact on property value

The presence of Japanese knotweed can reduce a property's value, sometimes significantly. Even after treatment, the stigma of having had knotweed can linger and affect marketability. Research by the Royal Institution of Chartered Surveyors (RICS) has estimated that Japanese knotweed can reduce property values by between 5 and 15 per cent, depending on the severity and location of the infestation.

Legal obligations

While it is not illegal to have Japanese knotweed on your property, it is an offence under the Wildlife and Countryside Act 1981 to cause it to grow in the wild. This means you have a responsibility to prevent it from spreading to neighbouring land. Failure to manage the plant can result in legal action from neighbours and potential liability for any damage caused to their property.

The TA6 property information form

When selling a property in England and Wales, you are required to complete a TA6 Property Information Form, which includes a specific question about Japanese knotweed. When remortgaging, the lender's surveyor will also look for evidence of knotweed during their inspection. Honesty and transparency about the issue are essential, as concealing the presence of knotweed can have serious legal consequences.

How Japanese Knotweed Affects Your Remortgage

The impact of Japanese knotweed on your remortgage depends on several factors, including the severity and location of the infestation, whether a treatment plan is in place, and the specific lender's policy.

The RICS risk categories

The Royal Institution of Chartered Surveyors has developed a risk categorisation system for Japanese knotweed that most lenders and surveyors use when assessing the impact on a property. The system classifies knotweed into four categories based on its proximity to the property:

• Category 4 — Japanese knotweed is present within the boundaries of the property but is more than seven metres from any habitable space. This is the lowest risk category, and some lenders may be willing to proceed without a treatment plan, though most will want to see evidence of management.
• Category 3 — Japanese knotweed is within seven metres of a habitable space but does not cause damage to any structure. Most lenders will require a treatment plan to be in place before they will consider lending.
• Category 2 — Japanese knotweed is causing damage to boundary walls, paths, drains, or outbuildings, or is within the curtilage and affecting the fabric of the property. Lenders will typically require a professional treatment plan and may want evidence of progress before proceeding.
• Category 1 — Japanese knotweed is causing serious damage to the main building structure. This is the highest risk category, and many lenders will decline to lend until the issue has been substantially addressed.

Knotweed on neighbouring land

Even if the knotweed is not on your property, its presence on neighbouring land can still affect your remortgage. If it is within seven metres of your property boundary, many lenders will treat it similarly to knotweed on your own land. You may need to demonstrate that the neighbour is managing the issue or that your own property is protected.

Lender policies vary

Different lenders have different policies regarding Japanese knotweed. Some will decline any property with active knotweed, regardless of the category. Others are more pragmatic and will consider applications where a professional treatment plan is in place and being followed. A few lenders have relaxed their policies in recent years as understanding of knotweed management has improved. A specialist broker who keeps up to date with individual lender policies is invaluable in this situation.

Treatment Plans and Insurance-Backed Guarantees

Having a professional treatment plan in place is the single most important step you can take to improve your remortgage prospects when Japanese knotweed is present. Most lenders who are willing to consider affected properties will require evidence of a treatment plan as a minimum condition.

What a treatment plan involves

A professional Japanese knotweed treatment plan is typically prepared by a specialist contractor who is a member of the Property Care Association (PCA) or similar recognised body. The plan will include a detailed survey of the infestation, an assessment of the risk to the property, a recommended treatment method (usually herbicide application or excavation), a timetable for treatment, and ongoing monitoring arrangements.

Treatment methods

The two main approaches to Japanese knotweed treatment are herbicide application and excavation.

Herbicide treatment involves the systematic application of glyphosate-based herbicide over several growing seasons, typically three to five years. This is the most common and cost-effective method, though it requires patience as the plant is gradually weakened and killed over multiple treatment cycles.

Excavation involves physically digging out the knotweed rhizomes, which can extend to a depth of three metres or more. This method provides faster results but is significantly more expensive and creates substantial quantities of contaminated soil that must be disposed of at a licensed facility. Some contractors use a combined approach, treating with herbicide first and then excavating the residual material.

Insurance-backed guarantees (IBGs)

An insurance-backed guarantee is a vital component that most lenders require before they will consider a property with Japanese knotweed. An IBG provides protection for typically 10 years, guaranteeing that if the knotweed returns despite treatment, the specialist contractor (or the insurer behind the guarantee) will carry out further treatment at no additional cost.

The IBG is transferable, meaning it passes to subsequent owners of the property, which is important for protecting the lender's security over the mortgage term. The guarantee is underwritten by an insurance company, so it remains valid even if the original treatment contractor ceases trading.

Choosing a treatment company

Select a treatment company that is a full member of the Property Care Association (PCA) and can provide an insurance-backed guarantee. Check that the guarantee is underwritten by a reputable insurer and that the company has a track record of successful treatments. Ask for references and case studies, and ensure the company will provide all the documentation that lenders typically require.

Finding Lenders Who Accept Properties With Japanese Knotweed

While Japanese knotweed narrows the field of available lenders, there are still viable options for homeowners looking to remortgage. Understanding the lending landscape and using the right strategies can help you find a competitive deal.

Lenders who may consider knotweed-affected properties

Several categories of lenders may be willing to consider properties with Japanese knotweed, provided a professional treatment plan and insurance-backed guarantee are in place. These include certain high street lenders who have updated their policies in recent years, building societies that take a more pragmatic approach, and specialist lenders who focus on non-standard or unusual properties.

The role of a specialist broker

A mortgage broker with experience in knotweed-affected properties is arguably your most important ally. They will have current knowledge of which lenders are accepting applications, what documentation each lender requires, and how to present your case in the most favourable way. Lender policies on knotweed can change frequently, so having someone who stays up to date with these changes is extremely valuable.

Product transfers

If your current lender already holds a mortgage on the property, a product transfer to a new deal may be possible without triggering a new valuation or knotweed assessment. This can be particularly advantageous if the knotweed was discovered or worsened after you took out your original mortgage. Not all product transfers avoid revaluation, so check with your lender or broker.

Preparing your application

When approaching lenders, have all your documentation ready and organised. This typically includes the specialist knotweed survey report, the treatment plan from a PCA-accredited contractor, evidence of treatment already carried out (including dates, methods, and photographs), the insurance-backed guarantee certificate, and a management plan for ongoing monitoring. Presenting a comprehensive, well-organised application demonstrates to lenders that you are taking the issue seriously and managing it responsibly.

Be upfront

Always be honest and transparent about the presence of Japanese knotweed. Attempting to conceal or minimise the issue can backfire badly if the surveyor identifies it during the valuation. Upfront disclosure, combined with evidence of professional management, is always the best approach and is more likely to result in a positive outcome.

Costs of Dealing With Japanese Knotweed

Understanding the costs involved in managing Japanese knotweed helps you plan your finances and evaluate whether remortgaging is the right decision at this time.

Survey costs

A specialist Japanese knotweed survey typically costs a few hundred pounds, depending on the size of the property and the extent of the infestation. This survey forms the basis of the treatment plan and is usually a prerequisite for any lending decision.

Herbicide treatment costs

Herbicide treatment over a three to five-year programme typically costs between one thousand and several thousand pounds, depending on the size of the infestation and the number of treatment visits required. Annual monitoring visits after the initial treatment programme add further costs but are essential for maintaining the insurance-backed guarantee.

Excavation costs

If excavation is required, costs can be significantly higher than herbicide treatment, potentially running into tens of thousands of pounds for large or deeply established infestations. The cost includes the excavation work itself, removal and disposal of contaminated soil at a licensed waste facility, and replacement of clean soil. This option is usually only considered when speed is essential or when the knotweed is very close to the property structure.

Insurance-backed guarantee costs

The cost of an insurance-backed guarantee is usually included in the overall treatment plan price quoted by PCA-accredited contractors. The guarantee typically covers a 10-year period and is transferable to future owners. Separate IBG costs, if applicable, are usually a few hundred pounds.

Impact on property value

The reduction in property value due to Japanese knotweed should also be considered as a cost, even though it is not a direct expense. A lower valuation means a higher loan-to-value ratio, which can affect the interest rates available to you. However, once a successful treatment programme is well advanced or complete, property values tend to recover.

Legal costs

If the knotweed has spread from a neighbouring property, you may have a legal claim for the cost of treatment and any diminution in value. Legal action can be costly and time-consuming, but it may be worth pursuing if the neighbour has been negligent in managing the plant. Seek specialist legal advice before proceeding.

Long-Term Management and Prevention

Successfully managing Japanese knotweed is not a one-off exercise. Long-term vigilance and ongoing management are essential to prevent regrowth and maintain both the value of your property and your mortgage eligibility.

Monitoring after treatment

Even after a successful herbicide treatment programme, regular monitoring is essential. Japanese knotweed rhizomes can remain dormant in the soil for many years, and regrowth is possible if any viable material remains. Most treatment plans include annual monitoring visits for several years after the active treatment phase, and this monitoring should continue for the duration of the insurance-backed guarantee period.

What to watch for

During the growing season (spring and summer), inspect your property regularly for any signs of knotweed regrowth. Look for the distinctive red or purple shoots that emerge in spring, the heart-shaped leaves that develop as the plant grows, and the characteristic bamboo-like stems. If you spot any regrowth, contact your treatment contractor immediately so they can take action under the guarantee.

Record keeping

Maintain comprehensive records of all surveys, treatment visits, monitoring inspections, and any regrowth incidents. These records are invaluable when remortgaging, as they demonstrate your ongoing commitment to managing the issue and provide the documentary evidence that lenders require.

Neighbour communication

If Japanese knotweed is present on neighbouring land, maintaining open communication with your neighbours about management is important. A coordinated approach to treatment is far more effective than treating your property in isolation, as the plant can easily spread back across boundary lines. If your neighbour is unwilling to act, you may need to implement barrier treatments or physical root barriers to protect your property.

Planning for future remortgages

When planning future remortgages, ensure your treatment documentation and insurance-backed guarantee are up to date and readily accessible. As the treatment programme progresses and the knotweed is brought under control, your remortgage options should improve. Over time, a well-documented history of successful management can significantly reduce the impact on lender decisions and property valuations.

Disclosure obligations

Remember that you have an ongoing obligation to disclose the presence of Japanese knotweed (current or historic) when selling or remortgaging your property. The TA6 property information form specifically asks about knotweed, and providing false information can result in legal action. Having a comprehensive treatment history and insurance-backed guarantee to present alongside the disclosure demonstrates responsible ownership and can reassure lenders and future buyers.