A Japanese knotweed survey, the seller's TA6 answers, and the lender are the three points where Japanese knotweed and property purchases collide. Buyers in England and Wales who skip any…
A Japanese knotweed survey, the seller's TA6 answers, and the lender are the three points where Japanese knotweed and property purchases collide. Buyers in England and Wales who skip any one of those can complete on a house they cannot mortgage, cannot easily resell, or later have to sue over. This is not a gardening article. It is what to do, legally, before you exchange.
Japanese knotweed property risk is real, but it is also often overstated in chain panic. The job is to put a Japanese knotweed survey and the paper trail in the file, then decide. Do not guess from a photograph in a WhatsApp group.
Why Japanese knotweed matters on a purchase
Caveat emptor still applies to most physical defects. Japanese knotweed is different because the Law Society's TA6 property information form asks the seller, in terms, whether the property is affected. A wrong "No" can support a misrepresentation claim after completion. A "Not known" pushes the investigation onto you.
Lenders care because they are taking the house as security. Many follow the RICS professional standard on Japanese knotweed, which uses management categories rather than the old rule of thumb that anything within seven metres was automatically toxic to a mortgage. A Japanese knotweed mortgage is often still available if there is a funded treatment plan and an insurance-backed guarantee from a competent contractor. Category and paperwork matter more than folklore.
How much does Japanese knotweed devalue property? There is no single percentage. It depends on visible growth, damage, neighbouring land, the treatment plan, and what a particular lender will accept. Anyone quoting a fixed "loss of value" figure as if it were a tariff is guessing. Ask the surveyor and the lender, in writing.
The TA6 question, and why "No" is a high bar
The TA6 form asks whether the property is affected by Japanese knotweed. The seller answers Yes, No, or Not known.
Guidance since the 2020 revision is strict. To answer "No", the seller should be certain that no rhizome is present in the ground of the property, or within three metres of the boundary, even if nothing is visible above ground. Rhizomes can sit dormant for years. Many sellers who are not botanists cannot honestly reach "No". "Not known" is often the accurate box. It is not a trick, and it is not a green light for the buyer to ignore the issue.
If the answer is "Yes", the seller should say whether there is a knotweed management plan and provide it. If the answer is "Not known", your solicitor should raise an enquiry, and you should consider a specialist Japanese knotweed survey rather than relying on a mortgage valuation.
Do not treat a seller's "I would have seen it" as a survey. Japanese knotweed damage to property is not always a cracked wall in the brochure photo. It can be a stand behind a shed, a treated crown under new turf, or growth on the neighbour's side of a fence.
Japanese knotweed survey: what buyers should actually commission
A lender's valuation is not a Japanese knotweed survey. Drive-by and desktop valuations miss plants. A RICS Home Survey (level 2 or 3) is better, but a surveyor who is not looking for knotweed can still miss a dormant patch.
If the TA6 is "Not known", if the garden looks disturbed, or if neighbouring land is overgrown, instruct a specialist. Search interest in "Japanese knotweed survey" is high for a reason: it is the document lenders and insurers understand. Ask whether the inspector is competent to produce a report a lender will read, typically aligned with Property Care Association (PCA) practice and the RICS categories.
A Japanese knotweed survey cost varies with the site. Do not pick the cheapest PDF if the lender will not accept the firm. The report should say whether knotweed is present on the property, on adjoining land, whether there is damage, and what management category applies. "Free Japanese knotweed survey" offers are usually lead-generation. Treat them as marketing.
If knotweed is found, a management plan plus an insurance-backed guarantee is what usually unlocks a Japanese knotweed mortgage. The plan should be capable of being assigned to you on completion. Your solicitor should check that, and who pays for remaining treatments.
RICS management categories (in outline) are how many valuers now describe the risk: damage to buildings at the top end; knotweed on the plot but no structural damage; knotweed on adjoining land close to the boundary; and knotweed further away. Lenders do not all treat those categories the same. Your Japanese knotweed mortgage report should be sent to the lender before you treat a valuation as "fine". Do not rely on the retired seven-metre rule as if it were still the only test.
Japanese knotweed on neighbouring land is still your problem if it can affect value or lending. You cannot treat the neighbour's garden without rights. You can, through your solicitor, ask what they will do, and you can ask whether an indemnity product is even relevant. Encroachment disputes are slow. They are a poor reason to exchange and "sort it later".
Japanese knotweed mortgage and the chain
Getting a mortgage with Japanese knotweed is a lender decision, not a solicitor's promise. Some lenders will not proceed on category A (damage to the dwelling or ancillary buildings) until a plan is in place. Others will lend on lower categories with a retention or with a guarantee. Ask the broker to put the report to the lender before you go unconditional.
Indemnity insurance is not a treatment plan. It may help with a neighbour's stand in some category C-type situations. It will not kill the plant, and it will not always satisfy a buyer when you come to sell.
If you are also selling, assume the next TA6 will be asked of you. Keep the survey, the plan, and the guarantee with the title pack.
What your conveyancer should do
On buying a house, knotweed sits with the enquiries, not with completion-day theatrics.
- Read the TA6 answer and every previous survey the seller offers.
- Raise specific enquiries on "Not known" or on any sign of treatment (dead canes, membranes, contractor stickers).
- Commission or review the specialist report before you exchange.
- Check property searches. They will not find knotweed. They still matter for other risks. Do not confuse a local authority search with a plant survey.
- Make any treatment plan, guarantee, and cost undertaking a condition of exchange where the lender requires it.
If the seller refuses a survey, that is information. You can walk away, renegotiate, or take a specialist report yourself. Completing "because the chain is tight" is how misrepresentation files start.
FAQs
Can I still buy if there is Japanese knotweed on the property?
Often yes, if the category, the plan, and the lender line up. Exchange only when those three agree.
Is the seller lying if they ticked "Not known"?
Not necessarily. The 2020 wording made an honest "No" hard. Your protection is the survey, not outrage at the box they ticked.
Does knotweed always damage the house?
No. Japanese knotweed property damage is a fact to be surveyed, not a slogan. Category A is the serious structural end. Many stands never reach it.
What to do next
Do not exchange on a "Not known" without a specialist report you and the lender can live with. If the TA6, the survey, and the mortgage offer do not match, stop and tidy the file. Blackstone's buying a property team can raise the enquiries and conditions before the chain talks you into guessing.
Disclaimer: This article provides general information only and does not constitute legal advice on any individual circumstances.
