Japanese Knotweed Insurance Backed Guarantee Explained: Terms, Transfers and Why Lenders Want One

Published by PCA Certified Survey Team • Updated October 2026

If you have dealt with Japanese knotweed while buying, selling or remortgaging, you have probably heard the phrase "insurance backed guarantee", or IBG. Lenders ask for them, solicitors check for them, and contractors advertise them. But what does an IBG actually do, and what should you check before relying on one?

This guide explains how knotweed guarantees work in the UK, the difference between a contractor's guarantee and an insurance backed one, typical guarantee terms, how they transfer when a property is sold, and why lenders care. It is general information; always read the actual policy wording.

What Is a Knotweed Guarantee?

When a specialist contractor treats knotweed, they will often give a guarantee that the treatment programme will be completed and that, if the knotweed returns within the guarantee period, they will come back and treat it again at no extra cost to the owner.

The weakness of a contractor-only guarantee is simple: it is only as good as the company giving it. If the contractor stops trading, the guarantee may be worthless.

What Makes It "Insurance Backed"?

An insurance backed guarantee is designed to protect against that risk. The contractor's guarantee is backed by an insurance policy, so if the contractor ceases trading during the guarantee period, the insurer arranges for the remaining obligations to be met, typically by appointing another qualified contractor to complete the work. Exact terms vary between schemes and policies, so check the wording.

Typical Guarantee Terms: 5 Years vs 10 Years

Knotweed guarantees are commonly offered over periods such as 5 or 10 years, though other terms exist. The period usually starts at a defined point, for example when treatment begins or when the programme is completed.

• Shorter terms (for example 5 years): may be suitable for smaller, lower risk cases and can cost less.
• Longer terms (for example 10 years): give more reassurance to buyers and lenders and are often requested in mortgage situations.
• What lenders want: lender requirements vary. Some specify a minimum term or that the guarantee is insurance backed and transferable. Your broker or lender can confirm.

When comparing quotes, check the guarantee start date, its length, what it covers, and the conditions you must meet to keep it valid.

Why Do Mortgage Lenders Want an IBG?

A lender is lending against the value of the property. Knotweed creates uncertainty about future cost and saleability. A professional management plan with an insurance backed guarantee reduces that uncertainty: the plant is being treated by a specialist, there are records, and if the contractor disappears there is still a route to completing the work. For many lenders, that combination makes the difference between declining and proceeding. Read more in can you get a mortgage with Japanese knotweed.

Transferring a Guarantee When You Sell or Buy

One of the most valuable features of a good knotweed guarantee is that it can usually be transferred to a new owner. For sellers, this makes the property easier to sell. For buyers, it means the protection continues after completion.

• Check whether the guarantee is transferable and whether a form, notice or fee is needed.
• Make sure your solicitor obtains copies of the guarantee and the policy document.
• Check the guarantee has been kept valid, for example that the treatment plan has been followed.
• After completion, confirm that the guarantee and policy are in the new owner's name.

If you are a buyer, see our step-by-step guide to buying a house with Japanese knotweed. If you are a seller, the TA6 form guide explains how to disclose knotweed and treatment history.

What Does an IBG Usually Cover, and What Does It Not?

Usually covered

• Completion of the agreed treatment programme if the contractor ceases trading.
• Re-treatment of regrowth in the treated area during the guarantee period, according to the terms.

Often excluded or limited

• Damage to buildings, paving or drains.
• Regrowth caused by later disturbance, such as digging foundations without informing the contractor.
• Knotweed spreading in from neighbouring land outside the treated area (depending on the terms).
• Areas not included in the original survey and plan.

If you plan building work, speak to the contractor first. Disturbing treated ground without following their advice can affect the guarantee.

How Much Does an IBG Cost?

Some contractors include the insurance backed guarantee in the price of their treatment programme; others charge it as an optional extra. As a guide only, where charged separately it is often a modest proportion of the overall programme cost. For full budgeting, see our Japanese knotweed removal cost guide.

Questions to Ask Before You Rely on a Guarantee

• Is the guarantee insurance backed, and who is the insurer or scheme provider?
• How long is the guarantee and when does it start?
• Is it transferable to a new owner, and how?
• What conditions must be met to keep it valid?
• What exactly is covered and excluded?
• Is the contractor a member of a recognised body such as the Property Care Association (PCA)?
• Will the guarantee meet my lender's requirements?

An insurance backed guarantee will not make knotweed disappear on its own, but combined with a proper survey and a professional treatment plan, it is one of the most useful pieces of paperwork a homeowner can have. Start with a specialist survey, see our treatment methods, or send us your details.

Common Misunderstandings About Knotweed Guarantees

• "The guarantee means the knotweed will never come back." Not quite. It means that, during the guarantee period and within its terms, regrowth in the treated area will be re-treated.
• "Any guarantee is good enough for my lender." Lenders vary. Some want specific features such as insurance backing, a minimum term, or a contractor who belongs to a recognised trade body.
• "Once treatment starts, I can dig the garden." Disturbing treated ground without advice can cause regrowth and may affect the guarantee. Always check first.
• "The guarantee covers the whole property." Usually it covers the areas surveyed and treated under the plan. New outbreaks elsewhere may need adding.
• "A transferable guarantee transfers automatically." Sometimes it does, but often there is a notification step. Do not assume; ask your solicitor to check.

What to Keep in Your Property File

Whether you are staying put or planning to sell, keep a single file with all your knotweed paperwork. It makes remortgaging and selling far smoother. As a minimum, keep the original survey report and site plan, the management plan, every treatment record or visit note, the guarantee certificate, the insurance policy document or scheme details, and any correspondence about transferring the guarantee or about knotweed on neighbouring land. Digital copies are fine, but make sure they are easy to find when your solicitor or lender asks for them.

If paperwork from a previous owner has gone missing, contact the original contractor; many keep records and can reissue documents. If the contractor has ceased trading and the guarantee was insurance backed, the scheme provider may be able to help.

Frequently Asked Questions

What is a Japanese knotweed insurance backed guarantee?

It is a contractor guarantee for a knotweed treatment programme backed by an insurance policy, so the work can still be completed if the contractor ceases trading during the guarantee period.

Is a 5 year or 10 year knotweed guarantee better?

Longer terms give more reassurance and are often requested for mortgage purposes, but requirements vary by lender. Check with your broker or lender.

Can a knotweed guarantee be transferred to a new owner?

Many can. Check the terms for any form, notice or fee, and ensure your solicitor obtains the guarantee and policy documents.

Does an IBG cover damage caused by knotweed?

Usually not. IBGs typically cover completion of treatment and re-treatment of regrowth, not property damage. Read the policy wording.

Do mortgage lenders require an insurance backed guarantee?

Many lenders ask for a professional management plan and an insurance backed guarantee when knotweed is found, though exact requirements vary.