Survey Found Japanese Knotweed: A Seller's Action Plan

Discovering that the buyer's survey has identified Japanese knotweed on or near your property is understandably alarming — it is one of the most feared common survey issues in UK property sales. Media coverage has created a perception that knotweed makes a property unsaleable, but this is not accurate. With the right approach, the vast majority of sales involving knotweed proceed to completion.

What matters is how you respond. For a broader overview of how surveys affect sales, see our seller's guide to property surveys. Acting quickly, getting professional advice, and putting a treatment plan in place demonstrates to the buyer and their lender that the issue is being managed responsibly.

Understanding the RICS classification

The Royal Institution of Chartered Surveyors (RICS) uses a classification system that categorises Japanese knotweed based on its proximity to the property and the risk it poses. This classification is what the buyer's surveyor will use, and it directly affects how mortgage lenders respond.

The seven-metre threshold is significant because the underground rhizome system of Japanese knotweed can extend up to seven metres from the visible above-ground growth. This is why lenders are concerned about knotweed within this distance even if no damage is currently visible.

Your action plan

Step 1: Get a professional knotweed survey

Commission a specialist knotweed survey from a member of the Property Care Association (PCA) Invasive Weed Control Group. This survey will confirm the identification, map the extent of the infestation, classify it according to the RICS categories, and provide treatment recommendations.

A specialist knotweed survey typically costs £300 to £500. Do not rely on the buyer's surveyor's assessment alone — general surveyors are not knotweed specialists, and a professional assessment may result in a different (and potentially more favourable) RICS classification.

Step 2: Start a treatment plan

The two main treatment methods for residential properties are herbicide treatment and excavation.

Herbicide treatment

• The most common approach for residential properties
• Involves applying specialist herbicide (usually glyphosate-based) directly to the knotweed, typically by stem injection
• Requires multiple applications over two to three growing seasons
• Most effective when started in spring or summer during active growth
• Cost: £2,000 to £5,000 depending on the extent

Excavation and removal

• Involves physically removing the knotweed and contaminated soil to a depth of at least three metres
• Faster than herbicide treatment — can be completed in days
• Contaminated soil must be disposed of at a licensed landfill site
• More disruptive and significantly more expensive
• Cost: £5,000 to £15,000+ depending on access and volume

For most residential sellers, herbicide treatment is the preferred option because of the lower cost. Our guide on when to fix versus reduce the price can help you decide whether to treat the knotweed yourself or adjust the sale price. The key consideration is that herbicide treatment takes time — if you are mid-sale and the buyer needs the treatment to be well advanced before their lender will release funds, the timeline may be tight.

Step 3: Obtain an insurance-backed guarantee

An insurance-backed guarantee (IBG) is the critical document that most mortgage lenders require. The IBG is issued by the treatment company's insurer and typically provides 10 years of cover. If the knotweed returns despite treatment, the insurance covers the cost of retreatment and any damage caused.

Key features of a mortgage-compliant IBG:

• Minimum 10-year term
• Transferable to subsequent property owners
• Covers retreatment costs if knotweed returns
• Covers damage caused by knotweed regrowth
• Issued by a reputable insurance provider (check the insurer is Financial Conduct Authority regulated)

The IBG typically costs £1,000 to £2,000 on top of the treatment cost. This is a necessary investment — without it, most mainstream lenders will not proceed.

Step 4: Update the TA6

Section 7.8 of the TA6 Property Information Form asks specifically about Japanese knotweed. You must disclose its presence honestly, including the RICS category, the treatment plan, and the IBG details. Failure to disclose known knotweed is misrepresentation and could lead to a legal claim under the Misrepresentation Act 1967.

If you disclosed "not aware" on an earlier version of the TA6 and the buyer's survey has now identified knotweed, you must update the form. Your solicitor will handle this.

Step 5: Negotiate with the buyer

With your specialist survey, treatment plan, and IBG (or confirmation that an IBG will be provided), you are in a position to negotiate constructively. The buyer may request a price reduction to reflect the cost of ongoing treatment or the perceived stigma. See our guide on average price reductions after survey for benchmarks.

Be aware that knotweed-related price reductions often exceed the actual cost of treatment because of the perceived stigma. A buyer may request £20,000 or more when the total cost of treatment and IBG is £5,000 to £7,000. Having your own professional documentation helps counter inflated requests.

Mortgage implications in detail

The buyer's ability to obtain a mortgage is often the deciding factor. Here is what the main lender categories typically require:

• High street lenders (Lloyds, NatWest, Barclays, HSBC, Halifax): Generally require a professional treatment plan with a 10-year IBG for Categories A, B, and C. Category D usually accepted without conditions.
• Building societies: Requirements vary. Some are more flexible than high street banks; others are stricter. Check with the specific lender.
• Specialist lenders: Some specialist and bridging lenders will lend without a treatment plan, though often at a higher interest rate or with a reduced loan-to-value ratio.

If the buyer's mortgage offer is conditional on a treatment plan and IBG, providing these documents promptly is essential to keeping the sale on track. In some cases, the lender may impose a mortgage retention until the treatment plan is well advanced.

If the knotweed is on neighbouring land

If the surveyor has identified knotweed on neighbouring land (RICS Categories C or D), the situation is more complex because you cannot control what happens on someone else's property.

• If the knotweed is within seven metres (Category C), contact the neighbour to discuss treatment. Document your communication in writing.
• If the neighbour is unwilling to treat the knotweed, contact your local authority — they have powers under the Anti-social Behaviour, Crime and Policing Act 2014 to issue community protection notices.
• Consider installing a root barrier on your boundary to prevent rhizome spread (cost: £1,000 to £3,000 depending on boundary length).
• Disclose the situation on the TA6, including any steps you have taken and any communication with the neighbour.

Typical costs summary

Keeping the sale on track

The key to selling a property with Japanese knotweed is demonstrating that the issue is being managed professionally. Buyers and lenders are not looking for the knotweed to be completely eradicated before completion — they need evidence that a credible treatment plan is in place with appropriate insurance backing.

Acting quickly, being transparent, and providing professional documentation puts you in the strongest possible position. The stigma around knotweed is often worse than the reality, and most sales involving knotweed do proceed to completion when the seller takes a proactive approach. For more on this topic, see our dedicated guide on selling a property with Japanese knotweed.

Frequently asked questions