Property Guide · 2026/27
Japanese Knotweed and Your Mortgage
Japanese knotweed rarely stops a UK property sale outright, but it can complicate mortgage lending and creates specific legal disclosure and liability issues for buyers, sellers and neighbours. This guide explains how it is assessed and managed.
How It Affects Mortgage Lending
Japanese knotweed can grow through weak points in structures and hard surfaces, and lenders are cautious about the risk it poses to a property's value and structural integrity. Most mainstream lenders will still lend where knotweed is present, provided a professional treatment plan with an insurance-backed guarantee (IBG) is in place — but the closer the growth is to the building and neighbouring boundaries, the more likely a lender is to require treatment to be completed, or a retention, before releasing funds.
Distance-Based Risk Categories
Surveyors commonly assess knotweed risk using distance-based bands broadly in line with RICS guidance, considering how close the growth is to habitable space, boundaries and neighbouring land:
- Growth within the boundary but away from the building: generally the lowest risk category
- Growth within around 7 metres of habitable space or a boundary: a management plan is usually recommended and may affect mortgage decisions
- Growth within, or emerging through, the structure itself: the highest risk category, often requiring treatment or excavation before a sale or lending decision can proceed
A RICS-qualified surveyor or specialist knotweed contractor should always confirm the category for a specific property.
Seller Disclosure on the TA6 Form
The standard TA6 Property Information Form used in English and Welsh conveyancing specifically asks whether the seller is aware of Japanese knotweed on the property. Answering incorrectly — even inadvertently — can expose a seller to a claim for misrepresentation if the buyer later discovers untreated knotweed, so sellers should check carefully and disclose any known history, even if treatment has already taken place.
How Knotweed Is Treated
Specialist contractors typically offer two main treatment routes:
- Herbicide treatment programme: repeated applications over roughly 3 to 5 years, cheaper but slower and requires ongoing monitoring
- Excavation and removal: physically digging out contaminated soil and rhizome material, faster but more expensive and disruptive, sometimes required where a sale is time-pressured
Insurance-Backed Guarantees
An insurance-backed guarantee (IBG) continues to cover a property against knotweed regrowth even if the original treatment company later ceases trading. Most mortgage lenders will require an IBG, alongside evidence of a professional treatment plan, before agreeing to lend on an affected property. Use our Mortgage Calculator once your position is confirmed to model repayments.
Liability to Neighbours
Simply having Japanese knotweed on your own land is not, by itself, a criminal offence. However, if it spreads onto a neighbour's land and reduces its value or usability, this can amount to the civil tort of private nuisance, and courts have awarded damages in successful cases along with orders requiring treatment. Separately, allowing the plant to spread into the wild, or disposing of contaminated soil incorrectly (it is classed as controlled waste), can constitute an environmental offence.