Japanese knotweed and buying a house

Japanese knotweed near a house does not automatically make it unmortgageable or unsellable. Since March 2022, RICS has required surveyors and valuers to assess knotweed using four management categories based on its actual impact, rather than the old blanket 7 metre rule. If knotweed is found, you will usually need a specialist inspection and, in higher risk cases, a professional treatment plan with an insurance-backed guarantee before your lender will proceed.

Key points

• The RICS professional standard on Japanese knotweed took effect on 23 March 2022.
• Knotweed is placed in management category A, B, C or D depending on its impact.
• Valuers must report knotweed seen within 3 metres of the boundary. The old 7 metre rule has gone.
• Sellers are asked about knotweed on the TA6 property information form.
• Treatment usually takes several seasons and should come with an insurance-backed guarantee.

Why is Japanese knotweed such a concern?

Japanese knotweed is an invasive plant that spreads through underground rhizomes and grows vigorously each spring and summer. It can push through weak points in paving, patios, drains and boundary walls, and established stands are hard to eradicate without professional treatment. It is not illegal to have knotweed on your land, but allowing it to spread onto neighbouring land can lead to legal claims, and it must be disposed of as controlled waste.

For buyers, the main concerns are lender restrictions, treatment costs, potential disputes with neighbours and the effect on resale value. In practice the risk to the house itself is usually lower than its reputation suggests. It rarely damages sound foundations, but it can damage lighter structures and makes gardens harder to use.

How does the RICS 2022 standard work?

The RICS standard replaced the 2012 information paper, which used a distance of 7 metres from the house as the trigger for concern. It introduced a decision tree that places infestations into four management categories:

For mortgage valuations, knotweed seen within 3 metres beyond the boundary must be reported to the lender, normally as category D. Knotweed seen further away does not automatically need reporting, although a widespread and clearly unmanaged infestation may still be flagged.

What will a survey tell you about knotweed?

Your Level 2 Home Survey or Level 3 Building Survey surveyor will look for knotweed in the garden and in neighbouring land visible from the property. If they see it, or suspect it, they will report it and usually recommend a specialist inspection. A lender's valuer, carrying out a mortgage valuation, will report it to the lender using the management categories.

A surveyor is not a knotweed specialist, and there are limits to what they can see:

• Knotweed dies back over winter, leaving brown hollow canes that are easy to miss.
• It may have been cut down, covered or treated shortly before the property went on the market.
• Rhizomes can extend underground well beyond visible growth.
• Neighbouring gardens may not be visible.

If you have any concerns, a specialist knotweed survey, ideally by a member of the Property Care Association's invasive weed control group, gives a much clearer picture.

What questions should you ask the seller?

The TA6 property information form asks the seller whether the property is affected by Japanese knotweed, and gives options of yes, no or not known. Your conveyancer should push for a clear answer. If the seller says yes, ask for:

• A copy of the management plan and the company that carried it out.
• The insurance-backed guarantee, and whether it transfers to you.
• Records of treatment visits and any monitoring.

If the seller answers "not known", that does not necessarily mean they are hiding something, but you should take extra care with your own checks.

How is Japanese knotweed treated, and what does it cost?

The two main approaches are:

• Herbicide treatment over several growing seasons, followed by a period of monitoring. This is the most common approach for residential gardens and is usually the least expensive.
• Excavation, where contaminated soil is dug out and either removed off site or buried and encapsulated. This is faster but considerably more expensive, so it is mainly used where land needs to be developed quickly.

Costs vary with the size of the infestation and the method. A herbicide programme for a typical residential garden might cost from several hundred to a few thousand pounds, while excavation can run well beyond that. Lenders generally want the work done by a reputable specialist under a written Japanese Knotweed Management Plan with an insurance-backed guarantee.

Should you still buy a house with knotweed?

Often, yes, provided you go in with your eyes open. Questions to ask yourself:

• Which management category has it been placed in? Category C or D cases usually have little effect on lending.
• Is there an existing treatment plan and transferable guarantee?
• Will your lender accept it, and on what conditions?
• Has the price been adjusted to reflect treatment costs and the possible effect on resale value?

If the knotweed is on a neighbour's land, ask whether it is being treated. You cannot force treatment yourself, but you should know what you are buying next to.

Knotweed found in a survey is a strong reason to renegotiate the price or ask the seller to put a plan in place before completion. If you want an independent surveyor to look at a property before you commit, you can get a fixed-price survey booked with a local RICS or RPSA regulated surveyor. See how it works for the steps.

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