A very invasive plant species that can crack masonry and disrupt foundations. It’s existence can literally make a property un mortgageable and therefore possibly not saleable.
This particular outbreak is in the garden of the house to the rear. All neighbours are aware of it’s existence and in fact the JKW here is in the process of ongoing specialist treatment until 2020.
There can be no guarantees the treatment will work. Questions to ask include: what is the impact on value? and if I buy the house will I be able to sell it again?