Buying a House with Japanese Knotweed

Written by Emma Lunn

23rd Aug 2021 (Last updated on 16th Feb 2023) 8 minute read

What is Japanese knotweed?

Japanese knotweed is a non-native weed that grows and spreads very 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.

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

If you do cut it down, you need to dispose of it responsibly. 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

How Much Does Japanese Knotweed Devalue a House?

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

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

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

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 problems. How each case fits into this classification will affect your mortgage chances.

If the knotweed falls into category 2 or 3, you may need a bigger deposit or 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.

Will My Property Survey Highlight Japanese Knotweed?

When buying any property, it is recommended that you arrange a survey. 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."

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.

Both a RICS Level 2 (Homebuyer) 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.

If you have any inkling that a property you plan to buy is affected by knotweed, you should get a separate Japanese knotweed survey.

2022 RICS Update

RICS published a new guidance note this year (2022) for chartered surveyors. This abolishes the "7 meter rule" used by surveyors and valuers when assessing the threat of knotweed to property.

Effective as of 23 March 2022, this guidance is a result of "an improved understanding of Japanese knotweed". The note states that; "Substantial structures on sound foundations are unlikely to suffer structural damage due to Japanese knotweed". However, the it is clear that knotweed can and will impact "amenity spaces". These include lawns, paths and driveways. Knotweed will also cause issues for buildings in poor condition or on compromised foundations.

Essentially, RICS are encouraging valuers and surveyors to consider potential Japanese Knotweed management outcomes, rather than building proximity rules. It is anticipated that mortgage lenders will revise their lending criteria.

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 completed by a Property Care Association (PCA) approved surveyor. They will assess the case on the above RICS categorisation.

Do not 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. 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

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

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 knotweed is not being treated, you could insist that the seller arranges and pays 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.

Once you own the home, 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.

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, your conveyancer can advise you on what to do.

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. 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 and cover up or conceal knotweed.

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.