Thanks for your question. I will try to help.
There is nothing legally preventing you from selling your house if it has knotweed, if someone is prepared to buy it with such.
Unfortunately, there is a question in the standard property information forms that any seller completes and passes to the buyer’s solicitors which asks about whether the property suffers from knotweed. So, you would have to confirm that it does, if you did not it would be misrepresentation.
It would then be for the buyer to decide whether or not to have it investigated. If the investigation confirmed that remedial works were required it would be for the buyer to decide whether to pay for the costs of this themselves or to seek a reduction in the purchase price from you.
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The buyer's solicitor would have to report it to the lender. The lender would them pass the issue to the underwriter.
The underwriter would the decide whether they would insit on works being carried out before or after completion of the sale. I would expect they would insist on a report by a contractor being carried out before lending though to determine risk.
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