Why land you are not buying can still matter
Japanese knotweed spreads below ground, and a boundary is not a barrier to it. What is visible on the other side of a fence is a poor guide to how far the root system has already travelled.
The consequences reach you in three ways even if the plant never crosses: a surveyor may qualify a report, a lender may impose conditions, and a future buyer's solicitor will ask the same questions you are asking now.
Who is responsible for dealing with it?
Not you, ordinarily. The land it grows on is someone else's, and the obligation to manage it sits with them. That is a genuine limitation on what you can achieve: you can ask, but you cannot compel a neighbour or a landowner to treat their land by buying the house next door.
Where treatment is already in place on the affected land, it is worth knowing who it binds, how long it runs and what happens if it lapses. Management of a nearby infestation is an ongoing arrangement rather than a resolved event, and its value to you depends on someone else continuing with it.
What will my lender make of it?
This is the point at which neighbouring-land knotweed becomes concrete, because lenders set explicit distances and they do not agree on them. One lender's instructions may engage where knotweed is within three metres of the boundary; another's at seven. The same infestation can therefore be immaterial to one lender and a condition of lending for another.
Where the distance threshold is met, lenders commonly rely on the valuer to assess whether the value or suitability of the security is affected, rather than deciding on the conveyancer's report alone.
What if it is on a development site?
On a new-build or estate the affected land is frequently retained land, a common area or another plot rather than anything inside your title. The question then is not what a seller knows but who owns the affected land, who is obliged to deal with it, and whether that obligation can be enforced by you once you own your plot.
Establish whether a formal management and treatment plan exists, who it binds and how long it runs; whether a guarantee benefits individual plot owners or only the site owner; and whether the cost of continued treatment falls to an estate charge.
A site-wide plan can be genuine and effective and still leave you dependent on someone else continuing to implement it. That dependence is not a reason to abandon a purchase. It is a reason to know who is responsible, for how long, and what happens if they stop.