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Buying a House with Japanese Knotweed

Emma Lunn

Written by

23rd Aug 2021 (Last updated on 4th Apr 2024) 9 minute read

Japanese knotweed is one of the UK’s most invasive plants. It can cause extensive property damage and is notoriously tricky to get rid of.

There are an estimated 1.4 million homes affected by Japanese knotweed in the UK. This is either by directly affecting the property or because it is on neighbouring land.

Mortgage lenders usually won’t lend on a property with an infestation of the plant. Allowance may be made if there is a professional treatment plan backed up by insurance in place.

If you have Japanese knotweed on your property or are thinking of buying an affected home, it’s important to know what you can do about it. This includes identifying it, treating it and knowing the laws around it.

  1. What is Japanese Knotweed?
  2. What Does it Look Like?
  3. Why Is It a Problem?
  4. How Much Does Japanese Knotweed Devalue a House?
  5. Will My Property Survey Highlight Japanese Knotweed?
  6. Laws Regarding Japanese Knotweed
  7. What Does a Japanese Knotweed Survey Entail?
  8. Can I get a Mortgage with Japanese Knotweed?
  9. What Can You Do About Japanese Knotweed?
  10. Japanese Knotweed Removal Cost
  11. What is an Insurance Backed Guarantee?
  12. What is The Responsibility of the Seller?
  13. Should I Buy a House with Japanese Knotweed?
  14. Finding a Surveyor
  15. Next Steps of Buying a House

What is Japanese Knotweed?

Japanese knotweed is a non-native weed that grows and spreads quickly. It was initially introduced to Britain as an ornamental garden plant.

In winter, the plant dies back to ground level. In early summer the stems emerge from deep underground and can reach 2 metres tall. Knotweed can grow 10cm a day in the summer. It can get into a property via small cracks in foundations and walls, drainage systems or pipes.

Environet has a heat map which shows the areas of the UK where Japanese knotweed is worst.

What Does it Look Like?

Japanese Knotweed has a few distinctive features, that allows you to identify it. These include:

  • Tall, hollow and green cane/bamboo-like stems
  • Shovel-shaped green leaves
  • Small creamy white flowers through the summer
  • Roots that are dark brown on the outside and orange/yellow inside

Why Is It a Problem?

Japanese Knotweed can cause a range of problems for property owners. Not only can it damage your property, but in extreme cases, it can make the home unmortgageable. The plant itself is not dangerous to humans or animals, but it can cause issues to homes and structures.

Knotweed gets into structures by targeting weak points, such as cracks in masonry, and tries to grow through them. In doing so, it further weakens a building and causes additional issues.

Japanese Knotweed can cause damage across:

  • Your home and neighbouring properties
  • Retaining walls
  • Garden walls and fences
  • Outhouses and garden buildings
  • Tarmac, drives and paved areas
  • Drainpipes and underground infrastructure

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How Much Does Japanese Knotweed Devalue a House?

According to Gloverpriest Solicitors, Japanese knotweed can reduce the value of your home by 5-15%.

Japanese knotweed can devalue your home because:

  • It can damage your property and surrounding structures
  • If left untreated it can spread and grow very quickly
  • It is expensive to treat
  • Digging through dormant knotweed to build an extension can cause it to start growing again
  • Mortgage lenders are reluctant to lend on properties with knotweed infestations
  • It could lead to disputes with neighbours
  • It can affect the aesthetics of your garden

To learn more, read what to do if a surveyor devalues house.

Will My Property Survey Highlight Japanese Knotweed?

When buying any home, it is recommended that you arrange a property survey. A surveyor can tell you if Japanese Knotweed is present around the home and grounds. They will also inform you if it is on neighbouring land.

According to Environet; "Residential surveyors have a duty of care to both the homebuyer and the mortgage lender to identify Japanese knotweed during a survey, even if the seller has attempted to hide it."

Both the Level 2 HomeBuyers Survey and Level 3 Building Survey will highlight any presence of Japanese Knotweed. In Scotland, the Single Survey is approximately the same as the Level 2 survey. This will also flag any knotweed concerns.

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Laws Regarding Japanese Knotweed

It is not illegal to have Japanese Knotweed on your property. It is, however, illegal for you to cause it to spread outside the boundaries of your land.

Waste material (e.g. cuttings) from these plants is classed as ‘controlled waste’ under the Environmental Protection Act 1990. It must be disposed of at a permitted waste site.

Japanese knotweed is listed in Schedule 9 of the Wildlife and Countryside Act 1981 and subject to Section 14 of this Act. This states that it is an offence to plant or cause it to grow in the wild.

The Anti-social Behaviour, Crime and Policing Act 2014 means you are obliged to control invasive plants on your property. You could be fined up to £2,500 if you don’t take action to control knotweed.

What Does a Japanese Knotweed Survey Entail?

In addition to the standard property surveys offered by surveyors, you can also arrange specialist surveys. A Property Care Association (PCA) approved surveyor will need to complete a Japanese knotweed survey. They will visit the property and assess the case on the following categories:


What it means


Action: Japanese knotweed is present and is causing visible material damage to a significant structure. This is likely to affect value because repair and remediation costs will be incurred.


Action: No material damage to structures, but Japanese knotweed is likely to prevent use of or restrict access to amenity space. This may still affect value, but that will be related more directly to the cost of remediation because no structural repairs will be needed.


Manage: Japanese knotweed is present, but it is not causing damage or affecting amenity. Consequently, the impact on value will be much lower because the structures and amenity of the property have not been adversely affected. Any remediation costs will be at the discretion of the owner.


Report: Japanese Knotweed was not seen on this property, but it can be seen on a neighbouring property or land where it is within 3 metres away from the boundary.

The above categorisation came into effect in March 2022. This was following a revision of RICS Japanese Knotweed standards and guidance.

After the survey, you will receive a written report and recommended Knotweed treatment plan. If a firm offering treatment conducts your survey, it will include a detailed quote.

Japanese Knotweed Survey Costs

On average, a Japanese Knotweed Survey costs in the region of £200-£300+ VAT*. The survey cost will vary depending on location and size of the property/land being inspected. It will also depend on the firm you use.

You can get a very basic survey for free from some Knotweed treatment firms, but these are merely to serve as a quote for treatment.

*Based on costs from Taylor Weed Control, Knotweed Services and Japanese Knotweed Plus.

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Can I get a Mortgage with Japanese Knotweed?

Mortgage lenders differ in how they view Japanese knotweed. There isn’t an industry standard so each lender will have its own criteria.

However, the RICS categories in the section above can give you an idea of whether knotweed will impact your mortgage application. If the knotweed falls into category A or B, it will be harder to obtain a mortgage. RICS states “Most lenders are expected to impose retentions on mortgage advances pending receipt of a remediation specialist's report.”

No retention is imposed for category C. Likewise with category D, as it is on adjoining land and out of the control of the mortgage applicant.

What Can You Do About Japanese Knotweed?

According to the government website, you should not try to treat Knotweed yourself. Getting rid of knotweed is difficult and if you cut it, it will simply grow back. Standard weed killer won’t kill it either.

If you do cut it down, you must dispose of it responsibly. Disposing of it in your green waste or the wild is against the law.

The best course of action is to contact Japanese Knotweed specialists. They can advise you on the best course of action and create a treatment plan.

What is a Japanese Knotweed Treatment Plan?

A knotweed specialist can put together a Knotweed Management Plan (KMP). The elements of the plan will depend on the extent of the knotweed problems, but the plan is likely to include:

  • A specialist Japanese knotweed survey, report and site plan
  • Herbicide treatment visits
  • Cutting and removal of knotweed brown stems in winter
  • Monitoring visits
  • A guarantee which starts after two years of no regrowth and ends after 10 years

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Japanese Knotweed Removal Cost

Treatment and removal of knotweed can cost anything from a few hundred pounds to £15,000 or more. Exactly, how much it will cost depends on:

  • The size of the area affected
  • The extent of the growth
  • The location
  • Type of treatment required

    What is an Insurance Backed Guarantee?

    Most knotweed treatment companies will offer an insurance-backed guarantee. This promises to retreat any regrowth free of charge during the guarantee period.

    This is also known as:

    • Knotweed IBG
    • Japanese knotweed indemnity
    • Knotweed insurance-backed warranty

    Insurance-backed guarantees ensure that the knotweed management firm’s guarantee obligations are met. This is even if the company goes out of business.

    Most mortgage lenders will want this type of insurance in place for any property undergoing knotweed treatment.

    What is The Responsibility of the Seller?

    If your home is impacted by Japanese knotweed, it doesn’t necessarily mean you can’t sell it. However, you must be honest.

    Sellers must legally disclose if they know knotweed is present on their property. You may face legal action if they deliberately try to cover up or conceal knotweed.

    Legally, you need to state the property has knotweed on the TA6 Seller’s Property Information Form. You will also need to confirm if there is a knotweed management plan in place. If you lie on the form you could be sued for ‘misrepresentation’.

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    Should I Buy a House with Japanese Knotweed?

    If you are looking to buy a home that you believe has knotweed on the premises you should:

    • Arrange a RICS Home Survey Level 2 or Level 3
    • If knotweed is identified, get a Japanese Knotweed survey done
    • Ask the seller if a treatment plan with indemnity insurance is in place

    If the knotweed is not being treated, you could insist that the seller arrange and pay for treatment before you exchange contracts. However, some firms will only issue guarantees after completion of the treatment plan.

    Alternatively, you could re-negotiate the property price. This could account for the cost of paying for a knotweed management plan yourself.

    Finding a Surveyor

    Here at Compare My Move, we can connect you with up to 6 surveyors in the local area. We can also save you up to 70% on your overall surveying costs. Fill out our surveying form to be connected today and find the right company for you.

    All our surveying partners must pass our strict verification process. Companies offering Home Surveys and Valuation Reports must be registered with RICS. Firms offering Party Wall and Snagging Surveys can be regulated by the RPSA or RICS.

    Need a Removal Company?

    Once your survey and property transaction processes have been completed, you may need a removal company. Our integrated surveying form lets you request removal companies in just a few steps. We can also connect you with up to 6 removal companies and save up to 70% on your removal fees.

    Next Steps of Buying a House

    This article has been part of our home buying guide where we cover a range of topics to help you through the buying process. In our next article, we cover everything you need to know about buying a non-standard construction property. To find out more read buying and selling a non-standard construction house.

    Emma Lunn

    Written by Emma Lunn

    Freelance Personal Finance Journalist,

    Emma Lunn is an award-winning journalist who specialises in personal finance, consumer issues and property.