How Much Japanese Knotweed Devalues Property

Many factors can devalue the property of your house. Some are obvious, like the condition of the property and low energy efficiency. However, some factors are less obvious, such as the presence of Japanese Knotweed.

What is it, and why might it be a problem for house sellers?

We take a look in this guide about whether Japanese Knotweed devalues your property.

What is Japanese knotweed?

Native to Japan, Japanese knotweed is an extremely invasive plant known for causing considerable harm to buildings and infrastructure.

It’s a major worry for many property owners and presents notable difficulties in property transactions, whether buying or selling.

How does Japanese knotweed cause a problem?

Japanese knotweed is recognised as one of the UK’s most invasive and damaging plant species. It’s infamous for its uncontrolled growth, causing harm to buildings, taking over gardens and being notoriously hard to eliminate.

This aggressive plant can grow through concrete and tarmac, undermining the structural integrity of properties. If it gets out of control, its extensive root system invades drainage systems, which leads to costly repairs.

Beyond that, its presence in a garden can quickly overwhelm other plants, reducing biodiversity in the process.

The difficulty in completely eradicating Japanese knotweed only adds to the challenge and often requires professional intervention as well as ongoing management, often spanning years, to control its spread.

Lastly, due to its ever-spreading underground network of roots, it has the potential to infect neighbouring properties. This presents a risk of lawsuits from neighbours if they can prove that the plant has come from your property (see Network Rail Infrastructure Ltd v Williams & Waistell [2018] for a landmark case on this issue).

How does Japanese knotweed affect house prices?

Japanese knotweed can notably affect property values.

In most cases, you can expect the price to drop by between 5% to 20%, depending on the severity and proximity of the infestation. These are significant figures that could see many sellers selling at a deficit and making a loss.

The impact on your property’s value depends on several factors:

• Proximity to the building: The closer the knotweed is to the property, the greater the concern for structural damage
• Extent of the infestation: A small patch is less concerning than widespread growth
• Evidence of damage: Visible damage to structures, paths, or drainage increases the impact
• Treatment status: Properties with a professional management plan in place and an insurance-backed guarantee are less affected

Here’s what many sellers miss: mortgage lenders’ requirements changed in 2022.

The Royal Institution of Chartered Surveyors (RICS) updated its guidance in March 2022, moving away from the old blanket “7-metre rule” to a more nuanced risk assessment based on actual evidence of damage and interference (RICS, 2022).

This means lenders now assess knotweed on a case-by-case basis rather than automatically declining mortgages based on distance alone.

What does this mean for you as a seller?

If you have Japanese knotweed on your property, buyers will typically need to see:

• A professional survey from a qualified specialist
• A management plan outlining treatment
• An insurance-backed guarantee (usually 10 years minimum)
• Evidence of ongoing monitoring

Without these, most mortgage lenders won’t approve finance, severely limiting your pool of potential buyers to cash purchasers only.

How do I identify Japanese knotweed?

When buying or selling a property affected by Japanese knotweed, one major obstacle is its identification and the assessment of the infestation’s severity.

Despite its unique appearance with large, heart-shaped leaves and bamboo-like stems, it’s easy to mistake for other plants.

It’s also challenging to detect in its early growth stages, as it spreads underground.

Key identifying features:

• Stems: Hollow, bamboo-like stems with purple speckles
• Leaves: Large, shield-shaped leaves (up to 20cm long) with a flat base
• Flowers: Small creamy-white clusters appearing in late summer
• Growth pattern: Dense stands, often forming thickets
• Winter appearance: Dead, brown canes remain standing

If you suspect Japanese knotweed, it’s worth getting a professional survey. Misidentification can cause unnecessary alarm, while missing genuine knotweed can lead to legal problems down the line.

Eradicating Japanese Knotweed

Eradicating Japanese Knotweed poses a significant challenge due to its resilience.

Its extensive root system can reach several metres underground, complicating complete removal. The plant is also capable of regenerating from small root fragments, enabling it to rapidly reappear after removal efforts if they are not 100% thorough.

Consequently, professional services are required for effective treatment and eradication of the plant.

How much does Japanese knotweed cost to fix?

The sums involved in removing Japanese knotweed are substantial.

Treatment costs typically range from £4,000 to £40,000, depending on several factors:

• Size of the infestation: A small patch costs less than widespread growth across multiple areas
• Location: Urban sites with access restrictions or proximity to buildings cost more
• Treatment method: Herbicide treatment, excavation and removal, or root barriers each have different costs
• Site complexity: Presence of hard surfaces (tarmac, concrete) requiring breaking up adds to costs
• Guarantee requirements: Insurance-backed guarantees (essential for mortgage lenders) add to the overall cost

It can grow to over two metres high and spread quickly. If it gets out of hand, costs can reach tens of thousands of pounds.

How can I remove Japanese knotweed?

Due to the tough nature of Japanese knotweed, professional treatment is essential.

The most common treatment methods are:

1. Herbicide treatment: Chemical treatment applied over multiple seasons (typically 2-5 years for complete eradication). This is usually the most cost-effective method for residential properties.
2. Excavation and removal: Physical removal of the plant and its root system (which can extend up to 3 metres deep and typically up to 3-4 metres horizontally). This is faster but far more expensive and disruptive.
3. Root barriers: Installation of physical barriers to contain spread, often used in combination with herbicide treatment.

Whatever method you choose, make sure you get:

• A qualified specialist (look for Property Care Association members)
• A detailed management plan
• An insurance-backed guarantee (10+ years)
• Regular monitoring and follow-up treatments

These aren’t optional extras if you’re planning to sell. Mortgage lenders require them.

Can I use insurance to pay for Japanese knotweed?

Dealing with Japanese Knotweed? It might be worth checking your home insurance policy.

Most standard home insurance policies do NOT cover:

• Japanese knotweed treatment costs
• Damage to your property caused by knotweed
• Legal costs if neighbours sue you for encroachment

However, some insurers offer optional knotweed cover as an add-on, though you’ll need to be prepared for the possibility of increased premiums.

Important: If you know knotweed is present on your property and don’t disclose it when taking out or renewing insurance, any claim related to it is likely to be rejected.

It’s always best to be upfront and check your coverage details to avoid any unpleasant surprises.

Do I need to disclose Japanese knotweed when selling my home?

Yes, absolutely.

There are legal implications to consider if you’re a homeowner with Japanese knotweed on your property.

When selling your home, you’re required to answer honestly on the TA6 Property Information Form, which specifically asks whether you’re aware of Japanese knotweed on or near the property.

Failing to disclose can lead to serious repercussions:

• The buyer can sue you for misrepresentation after completion
• You could be liable for the buyer’s treatment costs
• You may face additional legal costs and damages

The TA6 form asks ‘Is the property affected by Japanese knotweed?’ with options for ‘Yes’, ‘No’, and ‘Not known’. You must only answer ‘No’ if you are absolutely certain that no knotweed exists above or below ground within 3 metres of the boundary. If you are simply unaware of any, you should answer ‘Not known’ to protect yourself from misrepresentation claims.

If you’ve had knotweed in the past but it’s been professionally treated, you must still disclose this and provide evidence of the treatment, management plan, and insurance-backed guarantee.

Additionally, if the knotweed spreads to neighbouring properties, you could face legal action from your neighbours.

So there’s more to think about than the immediate impact on your property — wider responsibilities and potential legal issues come with owning a property affected by this invasive plant.

What are the preventative measures for Japanese knotweed?

Regular garden maintenance is your best friend for stopping Japanese knotweed in its tracks.

Look out for the early signs and move quickly.

For example, if you spot it on a neighbour’s land, it’s wise to discuss it amicably. Early intervention can save a lot of hassle later.

Preventative steps:

• Regularly inspect your garden, especially near boundaries
• Familiarise yourself with what knotweed looks like in all seasons
• If you spot suspected knotweed, get a professional survey immediately
• Keep photographic records if you notice it appearing on neighbouring land
• Maintain good relationships with neighbours so issues can be discussed openly

Don’t let this pesky plant take over — stay vigilant and act promptly to protect your property and maintain good relations with your neighbours.

Can I sell my home with Japanese knotweed?

Yes, you can.

If you need to sell your home but have a Japanese knotweed problem, all is not lost.

However, your options will likely be limited:

Traditional sale (via estate agent):

• You’ll need a professional survey and management plan
• An insurance-backed guarantee (10+ years)
• Full disclosure on the TA6 form
• Expect the sale price to be reduced (typically 5-20%)
• Buyer will need mortgage lender approval (which requires all the above)
• Longer sales timeline due to additional surveys and lender reviews

Cash sale (fast house buyer):

• No mortgage lender to satisfy
• Faster completion timeline
• Buyer accepts the property as-is
• Price reflects the knotweed treatment costs
• No need for management plan to be in place before sale

Is it worth buying a property with Japanese knotweed?

From a buyer’s perspective, a property with Japanese knotweed can be worth buying — but only if the price reflects the reality of the situation.

What buyers should look for:

• A significant price reduction that covers treatment costs plus a buffer for risk
• A professional survey confirming the extent of the infestation
• An existing management plan from a qualified specialist
• An insurance-backed guarantee covering the treatment
• Evidence that treatment is progressing as planned

If the seller can’t provide these, buyers should factor in the full cost of professional treatment (£4,000-£40,000+) when making an offer.

For cash buyers and property investors, knotweed needn’t be a deal-breaker — it’s simply a factor that needs to be priced in.

For mortgage-dependent buyers, the lender’s surveyor will assess whether the knotweed management is adequate before approving finance.

FAQs

What is the 7 metre rule for Japanese knotweed?

The 7-metre rule is outdated.

Prior to 2022, mortgage lenders commonly used a blanket 7-metre rule: if Japanese knotweed was within 7 metres of a habitable building, the mortgage would typically be declined.

In March 2022, RICS updated its guidance to a more sophisticated risk assessment. The new standard introduced Management Categories (A to D) based on visible damage and actual interference with the property, rather than rigid distance metrics.

The current approach focuses on:

• Actual evidence of damage or interference
• Proximity (with 3 metres now considered the key threshold for concern)
• Whether a professional management plan is in place
• Whether an insurance-backed guarantee exists

This means lenders now assess knotweed on a case-by-case basis rather than automatically declining based on distance alone (RICS, 2022).

Is it hard to sell a house with Japanese knotweed?

Yes, it’s challenging to sell a house with Japanese knotweed due to its impact on property value and potential buyers’ concerns.

The main challenges are:

• Limited buyer pool (many mortgage lenders have strict requirements)
• Reduced sale price (typically 5-20% devaluation)
• Need for professional management plan and insurance-backed guarantee
• Longer sales timeline
• Full legal disclosure required

But you can always sell to a cash buyer like Property Rescue. Get your cash offer for your home now.

Is it a criminal offence to sell a property with Japanese knotweed?

No, it’s not a criminal offence to sell a property with Japanese knotweed.

However, sellers are legally required to disclose its presence honestly on the TA6 form.

Failure to disclose when you’re aware of it can lead to civil action from the buyer for misrepresentation, potentially resulting in you being liable for treatment costs, legal fees, and damages.

Under the Wildlife and Countryside Act 1981, it’s also an offence to cause Japanese knotweed to grow in the wild, though this rarely applies to domestic situations unless you’re deliberately spreading it or dumping contaminated soil.