What should the treatment documents consist of?
There are normally three separate things, and they do different jobs. A treatment plan setting out what was to be done and over what period. Evidence that the work was carried out and completed. And a guarantee covering the result for a stated term.
Who carried out the work matters as much as the documents. Lenders commonly require the contractor to be an accredited member of a recognised trade association, and a guarantee from a firm outside that framework may not be accepted even where the treatment itself was competent.
Treatment is also often a programme rather than an event. A plan that is still running is a different position from one that has been completed and signed off, and the documents should make clear which it is.
Will the guarantee actually pass to me?
A guarantee given to the current owner does not automatically benefit you. It needs to be transferable, and the transfer needs to happen. Where there is a mortgage, it will usually need to extend to the lender as well as to you and to anyone you sell to later.
The remaining term matters too. A ten-year guarantee issued eight years ago is not the same protection as one issued last year, and the point at which the term starts — completion of treatment, rather than the date of the plan — is worth establishing.
What will my lender want to see?
Lender requirements on treated knotweed are specific and they are not uniform. Minimum guarantee periods differ between lenders. So does whether the guarantee must be insurance-backed, property-specific and transferable to successors and mortgagees in possession.
The documents are also usually referred to the valuer rather than accepted on their face, so the timing matters: producing them late in a transaction can hold up an offer that everyone assumed was settled.
What if the guarantee paperwork is missing?
Someone saying treatment happened is not the same as evidence that it did. Where the plan, the completion evidence or the guarantee cannot be produced, the protection everyone has been assuming exists may not be available to you at all.
It matters which document is absent. A complete plan and completion certificate with no guarantee is a different problem from a guarantee with nothing behind it, and insurance backing determines whether the remedy survives the contractor going out of business.
Often the gap can be closed. The contractor may still hold records and be able to reissue or assign the documentation, and where the work was done under an insurance-backed scheme the administrator may hold the policy independently. That takes time, so it is worth starting as soon as the gap appears rather than at the point everyone wants to exchange.
If nothing can be produced, the position needs dealing with on its merits rather than by assuming the treatment was effective. A specialist can report on the current state of the ground, which is evidence about the property even though it is not evidence about the history. Lender requirements in this area are written around documents that in this situation do not exist, so the lender's position needs establishing early. None of this means the purchase cannot proceed — it means the risk is different from the one that was described.