Japanese knotweed: what it means when you are buying or selling a home

Few words on a survey report cause as much alarm as “Japanese knotweed”. Buyers have heard it can wreck a mortgage application. Sellers have heard it can wipe value off a house. Neither is quite right, but both fears are understandable, because when knotweed is handled badly it really can stop a sale.

The plant itself is a nuisance: a fast-growing, invasive weed with an underground root system that spreads and regrows from small fragments. What turns it into a property problem is how buyers, sellers, lenders and solicitors react to it, and when.

How knotweed comes to light

Knotweed usually surfaces in one of three ways:

• the seller declares it on the Property Information Form, which asks directly whether the property is affected
• the buyer’s survey or the lender’s valuer spots it, in the garden or close to the boundary
• it turns up next door, near enough to the boundary to raise the same questions

The form treats a property as affected if knotweed is within its boundaries or within 3 metres of them on neighbouring land, so knotweed in next door’s garden can be as relevant as knotweed in your own.

The Property Information Form, and why it matters here

If you bought your home ten years ago, you may never have seen this form. It went between the solicitors, and it is easy to forget it existed. Solicitors call it the TA6. It is the Law Society’s standard questionnaire, and it is the seller who fills it in: boundaries, disputes, alterations, guarantees, services, flooding and, among much else, Japanese knotweed. The buyer’s solicitor reads the answers before exchange, and the buyer is entitled to rely on them.

The form has changed. A new edition became mandatory for most conveyancing firms from 30 March 2026, designed to be completed once an offer has been accepted, and some sellers are now asked for much of the same information earlier, so their estate agent can show it in the property listing.

On knotweed, the form offers three answers: yes, no, or not known. To answer “no”, a seller has to be confident there is no knotweed in the ground, including roots with nothing showing above the surface, on their property or within 3 metres of the boundary. That is why “not known” is a common answer, and why it often prompts the buyer to ask questions of their own.

How surveyors and lenders assess it

Since March 2022, surveyors following the RICS guidance no longer rely on a simple distance rule. They place knotweed in one of four management categories, A to D, according to its actual impact: whether it is damaging a building, restricting use of the garden, manageable with treatment, or only present nearby.

That category matters, because lenders often base their response on it. At one end, many lenders will lend normally once a professional treatment plan is in place, often backed by an insurance-backed guarantee. At the other, where knotweed is damaging a structure or seriously restricting the garden, a lender may impose a retention or decline to lend until a remedial plan is agreed. Remember that the lender’s valuer works for the lender, not for you, as we explain in the mortgage valuation: what it is, and who it is really for.

If you are buying

Finding knotweed after your offer has been accepted raises three questions at once:

• Will my lender still lend, and on what terms? This depends on the lender, the category and whether a professional plan exists.
• Is the problem properly understood? A surveyor noting knotweed is not the same as a specialist assessing how far it extends and what treatment will take.
• Does it change what the house is worth to me? Treatment usually runs over several growing seasons, it affects how you can use the garden in the meantime, and you will be the one disclosing it when you come to sell.

Take a £550,000 house with knotweed found at the bottom of the garden, near the boundary. If a specialist confirms it is contained, a treatment plan with a guarantee is put in place, and the lender is satisfied, the purchase may well be able to go ahead. If the extent is unclear, the seller had answered “no” on the Property Information Form, and the lender will not proceed without a plan, the same discovery can put the whole purchase in doubt.

Being able to go ahead is not the same as it being wise to. Jon’s own view is cautious: he would not personally recommend buying a home affected by knotweed, even with a management plan in place, because the plan does not remove the cost, the limits on the garden while treatment runs, or the disclosure you will face when you sell. Plenty of buyers do proceed, and for some it is the right call. Whether to proceed, renegotiate or walk away is a judgement about evidence, cost and timing. We explain how survey-based renegotiation works, and why it so often goes wrong, in renegotiating the price after a survey.

If you are selling

If you know or suspect you have knotweed, the most important point is simple: answer the Property Information Form honestly. A false answer can give the buyer a claim for misrepresentation after the sale, which is far more expensive than dealing with the plant. If you genuinely do not know, the form allows you to say so, though that answer invites questions of its own.

The sellers who come through it best tend to have done three things before they go to market:

• had the knotweed assessed by a qualified specialist
• put a professional treatment plan in place, ideally with an insurance-backed guarantee that passes to the buyer
• made sure their estate agent knows, so that buyers hear about it early and calmly, not from their surveyor weeks later

Disclosed early, with paperwork in hand, knotweed becomes one fact among many. Discovered late, it looks like something was hidden, even when it was not.

When it is the neighbour’s knotweed

Knotweed close to your boundary on someone else’s land can affect a sale just as much as your own, and you have less control over it. The questions become who will treat it, whether a plan is already in place, and whether the neighbour will cooperate. The courts have held that knotweed spreading from a neighbour’s land can amount to a legal nuisance, but a dispute is rarely what anyone needs in the middle of a sale.

Is it illegal to have knotweed?

No. It is not an offence to have Japanese knotweed on your land, but you are responsible for stopping it spreading beyond your property, and causing it to spread into the wild is an offence. Soil or plant material containing it counts as controlled waste and must be disposed of through licensed routes, not with ordinary garden waste. The government’s guidance covers the detail.

Where an independent view helps

Jon has seen what happens to sales when knotweed is discovered late, and how differently the same problem plays out when it is handled early and openly. A consultation is a chance to look at the survey, the answer on the Property Information Form, the lender’s position and your timetable with someone who has no stake in the transaction going ahead, and to decide calmly what to do next.