Buying a House With Japanese Knotweed — The 2026 Rules

Japanese knotweed does not automatically stop you buying a house, and it has not done for some years. Most sales affected by it complete perfectly normally, with a treatment plan and a guarantee in place.

The bigger problem now is out-of-date advice. Almost everything written about knotweed still repeats the "7-metre rule" — the idea that knotweed anywhere within seven metres of a property is a lending problem. RICS replaced that in 2022. Surveyors now assess what the plant is actually doing, and the distance that matters for off-site growth is three metres, not seven.

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How surveyors actually assess it now

The 2022 RICS professional standard moved from a distance rule to an impact assessment. Rather than measuring how far the knotweed is from the house, the surveyor looks at what it is doing, and assigns one of four management categories.

• Category A — Action. Visible material damage to a significant structure. This is the serious one.
• Category B — Action. No structural damage, but the knotweed prevents or restricts use of amenity space — typically it has taken over part of the garden.
• Category C — Manage. Present, but causing neither damage nor loss of amenity. Impact on value is usually minimal.
• Category D — Report. Seen on neighbouring land within three metres of the boundary, with nothing on the property itself. Lenders typically apply no retention.

The change matters because the old rule caught a great deal of knotweed that was doing nothing at all. Under the current standard, knotweed growing quietly at the bottom of a long garden is a very different proposition from knotweed pushing at a conservatory footing — as it always should have been.

What lenders actually want

Not eradication. A treatment plan backed by an insurance-backed guarantee.

That last phrase is the one to hold onto. A treatment company's invoice proves you paid someone; an insurance-backed guarantee (IBG) means the remedial work is underwritten if the contractor ceases trading, and it is what a lender is looking for. It usually runs for ten years and, importantly, should be assignable — it transfers to the next owner, which protects your own resale.

Three things to check on any guarantee you are handed:

• Is it insurance-backed, or just the contractor's own promise?
• Is it assignable to you, and to whoever buys from you later?
• How many years are left? A guarantee with two years to run is worth much less than one with eight.

Treatment itself is normally a herbicide programme over three to five years rather than a one-off. Excavation and removal is faster and far more expensive, and is usually reserved for development sites or where building work is planned.

Who pays?

Whoever owns it when the survey lands — which in practice means it becomes a negotiation.

If knotweed is found after your offer is accepted, the realistic outcomes are that the seller arranges and pays for treatment before completion, that they reduce the price by the cost of the works, or that a retention is held from the mortgage advance until treatment is underway. Walking away is always available and occasionally right, but it is rarely necessary for Category C or D.

A price reduction should reflect the treatment cost plus any repair, not a vague discount for the word "knotweed". Get a quote from a professional treatment company so you are negotiating with a number rather than a fear.

If the seller did not tell you

This is where knotweed produces real litigation.

Sellers complete a TA6 Property Information Form, and question 7.8 asks specifically whether the property is affected by Japanese knotweed. Answering "no" while knowing otherwise is a misrepresentation, and buyers have recovered substantial sums for it — including cases brought years after completion.

"Not known" is a legitimate answer only where it is genuinely true. A seller who suspected and chose not to look is in a weaker position than one who never had reason to suspect anything.

If you discover knotweed after completion and believe the seller knew, keep the TA6 form, the survey and any photographs, and take advice quickly — these claims have time limits.

If it is on your neighbour's land

Knotweed encroaching from adjoining land can be an actionable private nuisance. The courts have accepted that the interference can be with the enjoyment and amenity of your land, rather than requiring physical damage to a building first.

In practice:

• Put it in writing, dated, with photographs. A neighbour on notice who does nothing is in a much worse position than one who never knew.
• Keep it proportionate. Most cases resolve when the neighbour arranges treatment; litigation is slow and expensive.
• Councils have powers. A community protection notice can be used where a landowner unreasonably refuses to deal with it.

Is Japanese knotweed illegal?

Having it in your garden is not an offence. Two related things can be.

Allowing it to spread onto other land. Under the Wildlife and Countryside Act 1981 it is an offence to plant or otherwise cause Japanese knotweed to grow in the wild.

Disposing of it wrongly. Knotweed and any soil containing its rhizome are controlled waste. It cannot go in your garden waste bin or to an ordinary tip, and moving it around can spread it — a fragment of rhizome is enough to start a new stand.

Frequently asked questions

Can you get a mortgage on a house with Japanese knotweed?

Usually yes. Most lenders will proceed where there is a professional treatment plan with an insurance-backed guarantee. The RICS management category assigned by the surveyor is what shapes the lender's response, with Category A the most difficult and Category D usually attracting no retention at all.

Is the 7-metre rule still used for Japanese knotweed?

No. RICS replaced it in 2022 with an impact-based assessment. The relevant distance for knotweed on neighbouring land is now three metres, and the focus is on whether it is causing damage or restricting use rather than on distance alone.

How much does Japanese knotweed treatment cost?

It varies with the size of the stand and the method. A herbicide programme runs over three to five years and is the usual approach; excavation is faster and considerably more expensive. Get a quote from a professional treatment company rather than working from a rule of thumb — you need the figure to negotiate with anyway.

Does Japanese knotweed devalue a house?

It can, but far less than the reputation suggests, and the effect tracks the management category. Where knotweed is present but causing no damage and no loss of amenity, the impact on value is usually minimal. Visible structural damage is a different matter.

Do I have to declare Japanese knotweed when selling?

Yes — question 7.8 of the TA6 Property Information Form asks about it directly. Answering dishonestly is a misrepresentation and buyers do successfully sue over it.

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General information on the law of England and Wales, not legal or mortgage advice. Management categories can only be assigned by a qualified surveyor inspecting the property, and lender policies vary and are applied case by case. Related reading: who organises a survey when buying a house, what to do when a buyer wants a reduction after the survey, and does home insurance cover subsidence.