Buying a buy-to-let property is about more than rental income. Checking its condition is just as important, and a survey may reveal unexpected problems.
One example is Japanese knotweed. This invasive plant can be expensive to remove and may make it more difficult to get a mortgage.
This article explains how Japanese knotweed can affect your property purchase, what questions to ask your surveyor and mortgage lender, and what steps you should take next.
What is Japanese knotweed?
Japanese knotweed (Fallopia japonica) is a fast-growing plant from East Asia. It was introduced to the UK in the 1800s as a decorative garden plant. It has bamboo-like stems, broad shovel-shaped leaves, and clusters of creamy-white flowers that bloom in late summer.
Now recognised as an invasive species in the UK, it is subject to legal controls. Under Section 14 of the Wildlife and Countryside Act 1981, it is a criminal offence to plant Japanese knotweed or otherwise cause it to grow in the wild. However, simply having it in your garden is not an offence.
Why does Japanese knotweed matter to a property investor?
Japanese knotweed spreads through underground stems known as rhizomes. Even tiny pieces can stay dormant for years before growing again.
As it grows, the plant’s stems can find existing cracks and make them wider. According to RICS, buildings with strong foundations are usually not at much risk. However, boundary walls, lighter structures, and paved areas are more likely to be damaged.
Besides causing physical damage, knotweed can also threaten your investment by:
- Restricting garden use: Growth may block access or limit how you can use outdoor spaces.
- Putting off buyers: Knotweed concerns may make the property harder to sell.
- Adding treatment costs: Specialist treatment can be costly and may take several years.
How do surveyors assess Japanese knotweed?
If a surveyor or valuer finds knotweed, they use a system set by RICS (the Royal Institution of Chartered Surveyors). Since March 2022, the RICS professional standard puts knotweed into one of four groups.
These categories depend on how close the knotweed is to the house and what effects it is having:
- Category A (Action): It is on the property and causing visible damage to a structure, such as a wall, garage or path.
- Category B (Action): There is no such damage, but it is likely to stop you using or reaching outdoor space, such as a garden or driveway.
- Category C (Manage): It is on the property, but it is not causing damage or affecting outdoor space.
- Category D (Report): It is on next-door land, within 3 metres of the boundary.
If the knotweed is more than three metres away from your property boundary, you usually do not need to tell your lender about it.
Whatever the category, RICS recommends getting advice from a specialist. They can confirm if it is knotweed, check how far it has spread, and make a treatment plan with costs.
How does Japanese knotweed affect property value?
There is no fixed amount that the value will drop. It depends on the knotweed category, any damage, the cost of treatment, and the local property market.
As a rough guide, RICS expects:
- Category A: Value is likely to fall, as you face repair costs as well as treatment.
- Category B: Value may fall, mostly in line with the cost of treatment.
- Category C: Any effect on value should be much lower.
- Category D: The effect is often small, unless the growth next door is severe.
Once you know the knotweed category and how much treatment will cost, compare these factors with the asking price.
If the price does not reflect the cost of treatment, you can offer a lower amount or ask the seller to pay for the treatment before you buy.
Can you get a mortgage on a property with Japanese knotweed?
Yes, in many cases you can. Having knotweed does not mean you will automatically be refused, but each lender has their own rules. The RICS category provides evidence for the lender, but it does not decide the outcome.
Here is what to expect, based on the RICS standard:
- Category A or B: Most lenders will want a specialist report and a plan to treat the plant. They will often want the work backed by an insurance-backed guarantee (IBG). These often last 5 to 10 years.
- Category C or D: RICS does not expect lenders to make treatment a condition of the loan. But some lenders still set their own, stricter rules.
For Category A or B, the lender might also use a retention. This means they hold back part of the loan until you show proof that treatment has started.
Lenders often want the work done by a firm that belongs to a trade body. The main ones are the Property Care Association (PCA) and the Invasive Non-Native Specialists Association (INNSA).
Let your mortgage adviser know about knotweed as early as possible, as a knotweed finding can slow your buy-to-let mortgage application. Reports take time. A retention can also leave you short of funds on completion day.
Could a bridging loan help if a knotweed report delays your mortgage?
If a knotweed report delays your buy-to-let mortgage, a bridging loan may help. It can fund a time-sensitive investment purchase while you arrange longer-term finance.
Before you consider a bridge, check whether the bridging lender will accept the property as security. You also need to review the total cost and your repayment plan.
Your exit strategy is how you plan to repay the bridging loan. This is usually done by refinancing to a traditional mortgage or selling the property. Use bridging as a step before longer-term finance, not as a solution to the knotweed problem itself.
What should you do if you are buying a property with Japanese knotweed?
If a survey flags knotweed or the seller discloses it, establish what it means for the purchase before exchanging contracts.
These five steps can help you understand the work, costs and funding involved:
- Arrange a specialist survey: Find out how far the knotweed has spread, what treatment is needed and how much it will cost.
- Confirm your lender’s requirements: Ask for written details of any reports, treatment or mortgage retention it requires.
- Review the paperwork with your conveyancer: Check the seller’s disclosures and request records of any previous treatment.
- Check any treatment guarantee: Confirm whether it is insurance-backed and can be transferred to you as the new owner.
- Revisit your figures: Factor in treatment costs and possible delays, then check that the purchase price, funding and repayment plan still work.
If you are investing from overseas and cannot visit, arranging your own condition survey is especially important. A mortgage valuation is for the lender’s benefit and is not a substitute for a survey of the property’s condition.
Proceed only once you understand the findings, know what your lender requires and have a clear plan to cover the costs and repay your borrowing.
How can GoGoProp keep the mortgage application moving?
If knotweed holds up your buy-to-let mortgage, a bridging loan from GoGoProp could fund your purchase in the meantime. This gives you time to meet the mortgage lender’s terms and arrange long-term finance. All loans are subject to approval.
Explore our bridging loans to see how they work and whether they could suit your purchase:
GoGoProp lends under Money Lending Licence No. 1341/2025.
Key takeaways
- Japanese knotweed can affect a property’s value and your mortgage application, but it does not always mean you will be refused.
- Each lender has their own criteria and may ask for specialist reports or a management plan.
- RICS categories show the reported impact, but they do not decide what the lender will do.
- A mortgage valuation is not the same as a specialist knotweed inspection or a full buyer’s survey.
- Ask for reports, treatment records, and guarantee details before you exchange contracts.
- Bridging loans may offer short-term finance if your mortgage is delayed, as long as the lender accepts the property and your situation.
Frequently asked questions
1. How much does a Japanese knotweed survey cost?
A specialist survey typically costs £200 to £600, and may be reduced or refunded if you book treatment. Lenders often require a firm registered with a trade body such as the Property Care Association (PCA).
2. Will Japanese knotweed devalue a property?
Knotweed can affect a property's value, but there is no set discount. The impact depends on the property, what is found, management costs, and local market conditions.
3. How much does it cost to remove Japanese knotweed?
Costs depend on the affected area and treatment method. Herbicide programmes typically cost £1,000 to £3,000 over several years, while excavation and disposal can cost £4,000 to £20,000 or more. Get a written specialist quote before agreeing a property price.
4. Can you treat Japanese knotweed yourself?
DIY treatment is difficult, and waste must go to a licensed site. Lenders often require specialist treatment with an insurance-backed guarantee, making a professional plan the safer choice for investors.
5. What banks give mortgages for Japanese knotweed?
Lender policies can vary and change over time. Ask a mortgage adviser to check the current criteria against the property’s reports and management plan.
