Buying A House With Japanese Knotweed On The Property


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Buying a house with Japanese Knotweed and indeed selling one with knotweed on the property can certainly be an unwelcome experience.

We often meet potential clients who’ve discovered Japanese Knotweed on their property and are distraught, often imagining disaster scenarios that will almost certainly never happen.

Just last week, we received a call from a house-owner who hadn’t slept properly or spoken to his partner for three days since discovering a very small infestation at the far end of their 60m garden!

We were able to reassure him that there was no realistic danger to the house, we could deal with the infestation inexpensively and any effect on the value of the property would be minimal.

With that being said, there are still many situations where Japanese Knotweed can be very expensive to deal with and very disruptive to those using the property; sometimes, there is a limit to the mitigation a contractor can provide.

Below are some of the most common questions we get from people considering buying or selling a property with Japanese Knotweed.


What’s the problem with buying a house with Japanese Knotweed ?

Buying A House With Japanese Knotweed

Buying A House With Japanese Knotweed On The Property Can Be Fraught With Issues


Houses can be devalued just by the presence of Japanese Knotweed but it can also damage the structure of buildings themselves or prevent large parts of the property being used.

Treatment costs are often not the biggest problem; left for too long, structures can become unsafe and need to be vacated during a repair or important areas such as gardens can be out of action during the summer growth season.

Infestations can be very difficult to get rid of with roots growing up to three or four metres deep in some cases and often extending several metres horizontally beyond visible growth.




We also find that it is unusual for established infestations to have a root system less than one and a half metres deep.

It is always advisable to consult a professional before you consider buying a house with Japanese Knotweed present.

How do I know if I need a management plan and insurance backed guarantee?

The Royal Institute of Chartered Surveyors (RICS) first released guidance for surveyors and mortgage companies in 2012, until then, mortgage companies would not lend against properties where Japanese Knotweed was present.

The 2012 guidance required any Japanese Knotweed on or within seven metres of the boundary to be put under a treatment programme with an insurance backed guarantee.

Although the old ‘seven metre rule’ reflected the potential horizontal annual spread of the plant, it was based on a worst-case scenario and focussed on the potential for damage to structures.

The old rule also did not take account of the difficulty, in some cases, of getting access to treat land owned by others.

The guidance was renewed in 2022 to focus both on the potential for damage to structures and the loss of amenity to the property; surveyors have also been given more discretion to advise on the course of action.

The new guidance was widely misreported in the media as being a complete relaxation of the requirements, which is still being quoted by some estate agents and even a few surveyors.

The general guide is that only infestations within the property or causing damage to the boundary structures will definitely require a management plan and insurance backed guarantee.

Any Japanese Knotweed within three metres of the boundary must be included in the surveyor’s report and monitored unless the surveyor advises that a management plan and insurance backed guarantee is required.

The monitoring requirement is not defined and will not necessarily be checked by the lender; although some lenders previously checked periodically on the progress of treatment works.

Where a management plan is required by lenders for infestations within three metres of the boundary, the insurer will require either a treatment programme extending outside of the boundary or the installation of a vertical Japanese Knotweed barrier on the boundary.

Can Japanese Knotweed damage the structure of buildings?

Yes, Japanese Knotweed is great at taking advantage of weaknesses in the materials or design of buildings. We’ve seen this plant force its’ way through small cracks in concrete and even pop out inside a house between the wall and skirting! Even sound brickwork can be broken apart and tarmac is no obstacle, as can be seen in a recent treatment below.


These pictures may look alarming but don’t despair, as a measured example, we’ve never seen Japanese Knotweed break through correctly designed, well laid concrete, that’s cast on a stable aggregate base.

Furthermore, even where a buildings’ design could make it vulnerable, once herbicide treatment begins, the plant tends to lose vigour and strength and begin to retreat, becoming a diminishing threat.

Why do house sales with Japanese Knotweed fall through so often?

We find that sales tend to fall through most often when the seller has not identified and/or declared the infestation up front and has not put a management plan and insurance backed guarantee in place.

The presence of Japanese Knotweed may be a ‘red-line’ for the prospective buyer who has no intention of buying a house with Japanese Knotweed on the property.

Often it’s simply the loss of trust that stops them going any further and they drop out immediately, wasting precious time for all involved.

Some buyers are put off by not being able to use the full garden during their first three or four summers.

Sometimes, owners ignore the problem, hoping to somehow avoid the cost of dealing with it but then the prospective buyers are put off by the potential delay, even though the infestation was not a great concern to them initially.

This is not specific to Japanese Knotweed, any factor that could lead to a delayed sale is best dealt with in advance; we often speak to annoyed estate agents who have lost patience with their sellers and now have little confidence in selling their property.

Can I be taken to court for having Japanese Knotweed on my property?

Whilst you can even intentionally grow Japanese Knotweed on your own property, if you ‘recklessly or intentionally’ cause Japanese Knotweed to grow in the wild, you can be prosecuted.

You can also be sued in a civil action if you allow an infestation to pass from your land onto a neighbouring property; a court case in 2017 known as ‘Williams and Waistell V Network Rail’ created a legal precedent when two neighbours successfully sued Network Rail.

They sued for treatment costs, devaluation of their properties and ‘loss of quiet enjoyment’ (amenity) of their property, as well as being awarded their costs.

Since then, a healthy ‘no-win-no-fee’ industry has sprung up with legal companies knocking doors to encourage people to sue their neighbours rather than resolving situations through dialogue.

At the same time, the industry has provided ‘light at the end of the tunnel’ for long-suffering property owners who have been affected by their irresponsible neighbours’ infestations.

Do I have to declare Japanese Knotweed when selling my property ?

It is a legal requirement to declare the relevant information in the ‘TA6 property Information form’, this includes the presence of Japanese Knotweed.

The question relating to whether Japanese Knotweed is present has three options; ‘yes’ ‘no’ or ‘don’t know’, but don’t be tempted to answer ‘no’ or ‘don’t know’ if you are aware that it’s present.

A buyer can sue the seller if they are not told but later discover it is present; a 2023 court case in the London area (known as Downing V Henderson) resulted in the seller having to pay around £200,000 in costs and penalties when the Judge refused to believe that they did not know it was there.

Most of our domestic Japanese Knotweed treatment contracts with a 10 year insurance backed guarantee cost between £2000 and £3000; considerably cheaper than losing a court case!

How much does Japanese Knotweed devalue properties?

Valuation of properties with Japanese Knotweed is not an exact science; in our experience the devaluation tends to range from 10 to 20% for the purposes of house sales, depending upon the level of impact the infestation will currently or may in the future.

A valuer will assess many factors such as the size of the infestation, proximity to the buildings or other structures, and whether a treatment plan and insurance backed guarantee is in place

We typically see that when the property valuation is being conducted for the purposes of a nuisance claim for damages, the devaluation tends to range from 20%, with a treatment plan and insurance backed guarantee in place, to 42% without a treatment plan and insurance backed guarantee in place.

Some property developers and investors we know will actually seek out properties with Japanese Knotweed because they can negotiate big discounts, especially where they are paying cash!

Should I sell my property with Japanese Knotweed at auction?

Many estate agents will pressure their clients to sell at auction; this may be easier and quicker for the estate agent to get their fee but it risks the seller losing out on a lot of money.

You can make it easier for your estate agent by declaring the knotweed up front with accurate information and by getting a management and treatment plan in place with a 10-year insurance backed guarantee.

How much does it cost to treat Japanese Knotweed?

Our charge for management and treatment plans with a 10-year insurance backed guarantee is dependant on many factors such as size of the infestation, future land use, previous treatment history and access.

Having said that, the majority of our domestic contracts fall between £2,000 and £3,000. When we dig out infestations, usually for building contractors who need the site clear of knotweed quickly, it can run into tens of thousands, largely due to the disposal costs for infested soil.

How long does it take to treat Japanese Knotweed?

The most common and cheapest method of dealing with Japanese Knotweed is by treating it with herbicide which usually takes us between two and six years, if it’s treated optimally.

There are other options that involve digging up the plant and root or rhizome but these are usually best carried out by a professional due to the risks involved in deep excavations.

Can I deal with Japanese Knotweed myself?

We have seen non-professionals successfully treat their infestation quickly in a few cases but we’ve also spoken to people who’ve been treating their infestation for 30 years!

If you’ve got years before you need to sell, it will not affect the use of your property and isn’t likely to cross to neighbouring land, then it may be worth having a go yourself but if you need it done as quickly as possible, your best bet is with an expert.

If you are buying a house with Japanese Knotweed present always take the time to get advice and even a free quote and survey from a certified professional.

It could save you much bigger headaches in the future !

James & James , Clearsafe

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