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Buying a House with Japanese Knotweed

Emma Lunn

Written by

23rd Aug 2021 (Last updated on 16th Sep 2021) 7 minute read

Japanese knotweed or ‘fallopia japonica’ isn’t just another type of plant – it’s the UK’s most invasive plant which can cause extensive damage to your property. It’s notoriously difficult to get rid of.

There are an estimated 1.45 million homes affected by Japanese knotweed in the UK, either because knotweed directly affects the property or because there is knotweed on neighbouring land.

Mortgage lenders usually won’t lend on a property with knotweed present or nearly unless 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 a home affected by knotweed, it’s important you know what’s involved.

This article will cover the following:
  1. What is Japanese knotweed?
  2. What Does it Look Like?
  3. How Much Does Japanese Knotweed Devalue a House?
  4. Can I get a Mortgage with Japanese Knotweed?
  5. What is a Japanese Knotweed Treatment Plan?
  6. Japanese Knotweed Removal Cost
  7. What is an Insurance Backed Guarantee?
  8. Should I Buy a House with Japanese Knotweed?
  9. Do Estate Agents Have to Declare it to Buyers?
  10. Will My Property Survey Highlight Japanese Knotweed?
  11. Can I Sell a House with Japanese Knotweed?
  12. Next Steps of Buying a House

What is Japanese knotweed?

Japanese knotweed is a non-native weed that grows and spreads very quickly. It was originally introduced to Britain as an ornamental garden plant, but it can damage property. It’s a strong clump-forming perennial, with tall stems.

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

Getting rid of knotweed is very difficult – if you cut it, it just grows back. Standard weedkiller won’t kill it. This is because it can grow from just a tiny fragment of rhizome (the underground root system), and it grows fast.

In the UK Japanese knotweed is classified as 'controlled waste' so if you do cut it down you can’t take it to your local tip to get rid of it. Disposing of it in the wild is against the law.

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:

  • creamy white flowers
  • bamboo-like quick-growing stems
  • shovel-shaped green leaves with red or purple flecks
  • roots that are dark brown on the outside and orange/yellow on the inside

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How Much Does Japanese Knotweed Devalue a House?

Japanese knotweed can devalue your home because:

  • it can damage your property or neighbouring properties
  • 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
  • it could lead to disputes with neighbours
  • it can affect the aesthetics of your garden

According to Japanese knotweed removal firm Environet, Japanese knotweed can reduce the value of your home by 10%.

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, RICS has five categories of knotweed problem and how each case fits into this classification will affect your mortgage chances.

Risk categoryWhat it means

1

No Japanese knotweed is found on the property or those nearby.

2

There is no Japanese knotweed on the property, but it has been found on a neighbouring building or land more than 7 metres away from the boundary.

3

Japanese knotweed has not been found within the boundaries of the property, but it is present on a neighbouring property within 7 metres of the boundary but more than 7 metres from the habitable spaces of the property.

4

Japanese knotweed has been spotted within the boundaries of the property but it is more than 7 metres from the habitable space. Any damage to outbuildings, paths and fences is only minor. A Japanese knotweed survey is required.

5

Japanese knotweed has been spotted within 7 metres of a habitable space at the property. This could either be within the boundaries of a property or in a neighbouring property and/or Japanese knotweed has caused serious damage to outbuildings, paths and boundary walls.

If the knotweed on or near your property falls into category 2 or 3, you might need a bigger mortgage deposit or to pay a higher interest rate on your mortgage.

If the property is in category 4 or 5 you are likely to be refused a mortgage. If you are offered a mortgage, it’s likely to be dependent on a Japanese knotweed treatment plan being put in place.

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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

A Japanese knotweed survey will need to be done by a Property Care Association (PCA) approved surveyor, who will assess it on the above RICS categorisation.

It’s a very bad idea to try and get rid of knotweed yourself – you need a professional.

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
  • extent of growth
  • location
  • type of treatment required

What is an Insurance Backed Guarantee?

Most knotweed treatment companies will offer an insurance-backed guarantee which 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

An insurance-backed guarantee ensures that the knotweed management firm’s guarantee obligation is met 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.

Should I Buy a House with Japanese Knotweed?

If you are thinking of buying a house with knotweed you should:

  • get a full building survey done by a RICS surveyor, including the garden
  • 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 property has knotweed but it’s not being treated, you could insist that the seller sets up and pays in full for a treatment program before you exchange contracts. However, some firms will only issue guarantees after completion of the treatment plan, which may be up to three years.

Alternatively, you could re-negotiate the property price, taking into account the cost buying a knotweed management plan and insurance guarantee yourself.

Once you own the property, 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.

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Do Estate Agents Have to Declare it to Buyers?

When you sell a property, you will need to complete a Seller’s Property Information Form or TA6 form. You must answer questions on this form honestly – there may be legal action if you lie.

The TA6 Form asks you to confirm whether your property is affected by Japanese knotweed. If it is, it will ask if there is a knotweed management plan in place.

If you are buying a property and the TA6 form states there is Japanese knotweed on the property, your conveyancer will advise you on what you should do.

Will My Property Survey Highlight Japanese Knotweed?

Sellers are legally obliged to declare it if they know knotweed is present on their property. Sellers may face legal action if they deliberately try and cover up or conceal knotweed.

Surveyors should be able to identify knotweed growing above ground during a full building survey. But it may be difficult to spot in winter.

If you have any inkling at all that a property you plan to buy will be affected by knotweed, you should get a separate Japanese knotweed survey done. Some knotweed surveys use specially trained dogs to detect knotweed, even if its hidden beneath the ground.

Can I Sell a House with Japanese Knotweed?

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

Legally, you need to state the property has knotweed on the TA6 Seller’s Property Information Form. If you lie on the form you could be sued for ‘misrepresentation’.

In most cases you’ll need to pay upfront for a knotweed management plan and insurance-backed guarantee in order to sell your home. Without this, you may be limited to cash buyers.

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.