Japanese knotweed
The survey did not lift a decking board. Why might a winter crown still be a limit rather than a clear garden?
Because knotweed dies back in winter to crowns at or below the surface, and a board the surveyor did not lift can hide that crown. Not looking under the deck is a limit of the inspection, not a finding that the deck is clear. The survey should say the boards stayed down, and you should not turn that limit into a management category or prise a board up yourself.
Last reviewed 24 October 2026. Written for buyers in England and Wales unless this answer says otherwise. No named surveyor has signed this page. It is general guidance, not a report on your house, and not legal advice.
What is the fuller explanation?
A winter visit already has less green growth to see. Add a deck and the ground under it is concealed. The surveyor can comment on growth at the edges, on damaged boards, or on nothing visible. They cannot report the soil they did not see. RICS knotweed categories are not awarded from a concealed area. Government guidance is that you must not cause Japanese knotweed to spread, and that soil and plant material are controlled. Lifting a board and digging a “quick look” is exactly how cuttings and soil get moved. Leave the deck intact until a specialist, if you need one, agrees a method with the seller.
Ask the surveyor where they could see the ground and where decking, paving or stored timber prevented it. If the seller says the deck was built over a treated area, ask for the plan, not for a lifted screw. A plan that does not mention the deck is not proof the deck was included. Your lender may want a specialist because of the limit. That request is reasonable to discuss. It is not a category. Do not let an agent call an unlifted deck “inspected and clear” because the visit happened to be in January.
What should you ask your surveyor?
- Which areas of ground were hidden by decking you did not lift?
- Did you see any growth at the edges of the deck, or none at all?
- Please confirm you are not giving a management category for ground under the boards.
What are the typical next steps?
- Do not lift boards or dig under them to answer the survey yourself.
- Tell the lender the deck was not lifted, if knotweed is already a question on this purchase.
- If a specialist is instructed, give them the limit in the survey rather than a storey that the garden was clear.
Costs vary with the building and with who you instruct. Where this page does not cite a published figure, ask the surveyor, a relevant specialist, or your solicitor for a written quote. Do not treat a forum average as the price.
Also asked about this
Is summer growth at the edge of a deck proof the boards hide a crown?
It is a reason to be suspicious. It is still not a category, and it is not a map of what is under the boards.
Does a new deck mean the knotweed was removed first?
No. It means there is a deck. Removal would be in a plan and in waste records, which you should ask for.