RICS risk categories
- Category 1: knotweed within 7m of boundary but not within curtilage — low risk, some lenders accept without a management plan
- Category 2: within curtilage but more than 7m from habitable space — management plan typically required by lenders
- Category 3: within 7m of habitable space, outbuilding, or boundary — high risk, lender refusal likely without completed management plan
- Category 4: causing or has caused structural damage — specialist structural assessment required; lenders may refuse even with a plan
Legal obligations
- Wildlife and Countryside Act 1981 s.14: criminal offence to cause knotweed to spread to neighbouring land
- Environmental Protection Act 1990 s.79: local authority can issue abatement notice if spread constitutes statutory nuisance
- Anti-Social Behaviour Act 2014: Community Protection Notices (CPNs) — breach carries fines up to £2,500
- Private nuisance (Network Rail v Williams 2018): neighbours can bring civil claims for encroachment even without physical damage
- Knotweed-contaminated soil and cuttings are controlled (special) waste — must be disposed of at licensed facilities
Disclosure obligations
- On sale: TA6 Property Information Form asks about knotweed — false answers are fraudulent misrepresentation
- On remortgage: surveyor may flag knotweed; lender may require management plan as mortgage condition
- To neighbours: voluntary early disclosure if knotweed has spread — demonstrates good faith, may prevent nuisance claim
- Insurance Backed Guarantee (IBG) from a specialist company is transferable to purchaser or lender
Treatment options
- Herbicide (glyphosate): 2-3 applications per season over 3-5 years — most common approach
- Excavation: fastest but expensive (£5,000-£50,000+), creates significant controlled waste, may disturb foundations
- Root barrier membrane: prevents regrowth into protected areas but does not kill the plant — used with herbicide treatment
- DIY herbicide rarely effective against established infestation — use INNSA-certified or BASIS-qualified contractors
- Professional management plans with IBG: essential for mortgage and resale purposes
Impact on mortgageability
Pre-purchase check
Always commission a RICS survey before acquiring a property in an area of known knotweed prevalence. Knotweed can be dormant (invisible) in winter — consider a specialist botanical survey if the risk is high.
- Negotiate purchase price reduction to cover management plan cost and IBG
- Buyer's solicitor may seek a retention until management plan is in place
- Category 3 or 4 knotweed without a management plan: may prevent refinancing, sale, or further borrowing for years