Japanese knotweed rarely brings down a house. It brings down sales, mortgage offers and sometimes a seller’s defence in court.
In 2023, a buyer clearing the garden of his new £700,000 home in south-west London found knotweed growing behind a bush near the shed. The seller had ticked “no” on the TA6, and the bill that followed came to roughly £200,000. The plant hasn’t changed much since then, but the form has, so here is where knotweed stands for conveyancers in 2026.
What is Japanese knotweed, and is it illegal to have it?
Japanese knotweed is an invasive plant that spreads underground through rhizomes, and even small fragments can regrow. Left unmanaged, it can push through paving, crack garden walls and get into drains and outbuildings. It is far less likely to threaten the structure of a sound house, although buyers and lenders don’t always find that reassuring.
Having it isn’t illegal, and there’s no duty to remove it from your own land. The law applies in three other places. It is an offence under section 14 of the Wildlife and Countryside Act 1981 to plant it or otherwise cause it to grow in the wild. Knotweed and soil containing it count as controlled waste, so it can’t go in the green bin (or a skip). And councils can serve a community protection notice on an owner who lets it spread and does nothing about it.
What does the TA6 ask about it now?
Since 30 March 2026, firms accredited under the Law Society’s Conveyancing Quality Scheme have had to use the sixth edition of the TA6 for every new instruction. The fourth and fifth editions have been withdrawn, although matters already running on an older form can carry on with it.
Knotweed sits in section 8, Environmental matters. Question 8.6 asks whether the property is affected, and 8.7 asks about surveys and management plans. The Law Society’s explanatory notes say a property is usually treated as at risk if knotweed is growing on it or within three metres of its boundary. They also accept that sellers can’t easily be certain, because rhizome can sit in the soil with nothing showing above ground. That’s why many sellers answer “not known”.
Is “not known” a safe answer?
If it’s true, yes. An honest “not known” passes the job of investigating to the buyer, who can commission a specialist survey if they want certainty.
It isn’t a hiding place, though. A seller with a treatment invoice in a drawer doesn’t “not know”, and the notes are clear that sellers must answer from what they know and update their answers if anything changes before exchange. That applies to executors and attorneys too, even if they’ve never set foot in the garden.
The riskier box is “no”, because it is a positive statement that the plant isn’t there. In the 2023 case, expert evidence showed the knotweed by the shed had once stood around two metres tall and had been treated with herbicide during the seller’s ownership. The court didn’t accept that he believed his answer, and he was ordered to pay around £32,000 in damages plus up to £95,000 of the buyer’s legal costs.
Whatever happened to the seven-metre rule?
RICS dug it up in 2022, and unlike the plant, it hasn’t grown back. The RICS standard Japanese knotweed and residential property, in force since 23 March 2022, replaced the old rule of thumb that any knotweed within seven metres of a house was a problem.
Valuers now use a decision tree that places an infestation in one of four management categories, A to D, based on what it is actually doing: whether it’s affecting a structure, restricting use of the garden or simply growing nearby. Lenders generally only expect a retention in the more serious categories, and knotweed on a neighbour’s land within three metres of the boundary won’t usually trigger one by itself. Individual lenders can still set their own criteria, so it’s worth checking.
What if it crosses the boundary?
Then it becomes a nuisance question. In Network Rail v Williams [2018] EWCA Civ 1514, the Court of Appeal held that rhizomes encroaching onto a neighbour’s land can be an actionable nuisance before any physical damage is done, where the landowner knows (or ought to know) about the risk and doesn’t act reasonably.
The Supreme Court narrowed that in Davies v Bridgend County Borough Council [2024] UKSC 15. Knotweed had spread from council land into Mr Davies’s garden long before the council was in breach of any duty, and he couldn’t recover the loss in value that remained after treatment, because the breach hadn’t caused it. Historic spread is now a much harder claim to win. Knotweed heading over the fence today is a different matter.
What should a buyer ask for?
Paperwork. Where knotweed is present or has been treated, the buyer’s conveyancer should expect to see:
- a survey by a contractor accredited by the Property Care Association or the Invasive Non-Native Specialists Association
- a written management plan, usually running over several years
- an insurance-backed guarantee, typically for ten years, with confirmation that it transfers to the buyer
- records of the treatment carried out so far
The aim these days is management rather than eradication. A documented, guaranteed infestation is something a lender can work with. An undocumented one is a guess. An environmental search can also help show whether knotweed has been recorded nearby. [CHECK: confirm which OneSearch reports include knotweed data and link the specific product]
The knotweed behind that shed in south-west London cost its seller about £200,000, and hardly any of it went on digging. Almost all of it came from one tick in the wrong box.
So treat question 8.6 as the start of a conversation. A “yes” should arrive with a plan and a guarantee, a “not known” is an invitation to survey, and a “no” deserves a polite question about what it’s based on.
