Buying a property with Japanese knotweed

You can buy a house with Japanese knotweed, but you should understand what you are taking on before you exchange contracts. Check the seller's TA6 answers, the RICS Management Category in your survey, and whether a management plan with a transferable, insurance-backed guarantee is in place.

The seller's TA6 answers

Most standard residential sales in England and Wales use the Law Society's TA6 Property Information Form. Its 6th edition (2025), mandatory for Conveyancing Quality Scheme (CQS) firms on new transactions where the solicitor was instructed on or after 30 March 2026, covers knotweed in questions 8.6, on whether it is present, and 8.7, on any survey or management.

The Law Society's notes say you can rely on the seller's answers and may be able to claim compensation after completion if they are misleading. Sellers should attach any survey report or management plan and must say if they do not know an answer. Many answer "not known" because rhizome can be underground with nothing visible, so treat that as a reason to look harder, not as a no.

A "yes" may refer to knotweed that has already been treated, because RICS notes that sellers still have to declare it after remediation. Ask for the paperwork either way.

The survey

Whatever the type of survey, RICS surveyors use the same decision tree to categorise knotweed, and inspect the grounds visually from within the boundaries and, where necessary, from adjoining public property. Only the depth of inspection differs.

For specific advice on whether knotweed is present or how to deal with it, the standard says most surveyors should recommend a specialist from a recognised trade body.

Limits of an inspection

RICS accepts that assessments reflect only what is visible at the time, and that a competent inspection may still miss knotweed. It dies back between September and November, and although the brown canes usually stay standing through winter, the PCA says spotting it is hardest if they have been blown down or cut and cleared. It also notes that concealment under tarpaulin, fabric, gravel or bark is common.

Treatment can also hide it. Some herbicide treatments make rhizome go dormant, RICS says rhizome can lie dormant for decades, and later disturbance such as digging foundations can trigger regrowth. Treated knotweed may regrow only a few centimetres tall with small, elongated leaves. Small stands, dense planting, high fences and hard-to-see boundaries also limit what a surveyor can check.

RICS categories and lending

A surveyor or valuer who sees knotweed places it in one of four Management Categories under the RICS standard, in force since 23 March 2022.

These are RICS expectations, not lender rules: each lender sets its own mortgage policy on knotweed, some make no retentions at all, and a lender's instructions to its valuer take precedence over RICS guidance.

Documents to request

Ask your conveyancer to request:

• the survey report, and who produced it
• the Japanese Knotweed Management Plan (JKMP), which RICS says should describe the infestation with a scaled plan, give the contractor's details and methods, and include a treatment schedule updated as work is done
• any completion certificate, which follows a two-year "no growth" period
• the guarantee, which should last at least five years after completion, with insurance that keeps it valid if the contractor stops trading
• confirmation that the plan transfers to later owners and covers the whole property, and of who has paid for the work

As of 2022, RICS names the Property Care Association (PCA) and the Invasive Non-Native Specialists Association (INNSA) as the recognised trade bodies, and PCA membership can be checked on the PCA member search. Herbicide control needs at least four years of treatment and monitoring before a certificate, so a plan may still be running when you buy. Lenders take reassurance, RICS adds, when the current owner has paid the costs up front.

Where pre-contract enquiries reveal knotweed the surveyor cannot see, the standard says they should recommend that you or your legal adviser inspect these documents, or recommend further investigation if there is no satisfactory evidence of a planned programme.

Negotiating the price

There is no fixed percentage reduction. RICS tells valuers not to apply a standard percentage, nor to reflect only the cost of remediation, but to weigh five factors: the impact on the market before remediation, restrictions on use, the impact during remediation, any infestation on adjoining land, and the impact on saleability after remediation.

By category, RICS says value is likely to be affected in A and may be affected in B. In C any effect is usually modest and mostly reflects remediation cost, and in D it is usually limited unless a severe neighbouring infestation makes expensive defensive measures desirable. Some stigma is expected to remain for a period after treatment, though it should reduce over time.

A specialist's quote gives a firmer basis for negotiation. The PCA's guideline for valuers puts a residential herbicide programme for 1 to 25 m² at £2,000 to £4,000, noting that site-specific constraints can push costs higher, and the cost calculator gives an indicative range.

Knotweed on neighbouring land

For Category D knotweed on adjoining land, RICS says lenders should not make remediation a mortgage condition, because the owner cannot control it. The risk of spread depends on how extensive the neighbouring infestation is, and a specialist may advise on defensive measures. Knotweed more than 3 m beyond the boundary is normally just noted, unless it is a large, mature, unmanaged stand on land such as a derelict site.

In Network Rail v Williams and Waistell (2018), the Court of Appeal held that rhizome spreading onto a neighbour's land is an actionable nuisance even without physical damage, where the owner knew or ought to have known of the risk and failed to take reasonable steps. A fall in value caused only by knotweed next door is not, on its own, actionable. In Davies v Bridgend County Borough Council (2024), knotweed had spread onto the land before Mr Davies bought it, and the Supreme Court refused damages for the fall in value because it happened before the council's breach began.

If knotweed is already near the boundary, take advice before you buy rather than relying on a claim against a neighbour later.

Specialist surveys

Consider a specialist knotweed survey before you exchange if:

• the seller answers yes or "not known" on the TA6
• your survey reports knotweed in any category, or you spot something you suspect
• knotweed is close to a boundary (for Category C, RICS says surveyors should recommend a specialist inspection to non-lending clients, particularly within 3 m of a boundary)
• there is no management plan or guarantee, or it may not transfer to you
• your lender asks for a specialist inspection

The photo check gives an indicative view of a plant you are unsure about, but is no substitute for a specialist survey. If you need one, we can introduce you to up to three independent specialists who cover the area.