Unfortunately, it's about as big as it gets.. That is if it is not dealt with correctly by you, and explained by your agent. Japanese Knotweed is one of the most, if not the most, invasive plant species in the country, capable of doing all sorts of serious damage to buildings.
So, if you have discovered it in your garden, then you really need to start taking the necessary steps to have it eradicated immediately - and certainly before you actually put your home on the market.
Why the urgency? Because a property with untreated Japanese Knotweed is effectively unsellable - to anyone other than a cash buyer. To make it plainer still, the great majority of lenders will simply refuse to grant a mortgage on any knotweed-affected property where a recognised professional treatment and eradication programme, backed by a suitable insurance-backed guarantee for a minimum of 10 years, has not already been put in place, and paid for by the vendor. And in particularly serious cases, for example, where the stuff is found growing within 7 metres of the actual property (the distance which the root system can spread from the nearest visible stem), some lenders simply won't touch them, even if they have been properly treated.
In some instances homeowners claim to have successfully dealt with the problem themselves. And maybe they have. But that won't help them if they ever try to sell. They will be legally obliged to declare in the Vendor's Questionnaire that the property has had knotweed, and thereby reveal that they have not had it dealt with professionally. That will be enough to make just about every lender run a mile!
So what does a professional treatment and eradication programme involve? It will generally start with an expert evaluation of the extent of the problem, followed by at least three annual treatments with powerful herbicide. But that's not all. The contaminated soil will quite probably also have to be professionally removed, often down to a depth of 2 metres. And since it, together with all parts of the plant itself, is classed as controlled waste, this will all have to be disposed of according to the appropriate Environment Agency regulations, at a facility specifically licensed to deal with it.
To conclude we seriously recommend that you don't waste any time. Get a specialist firm in to evaluate the problem, and begin the process of eradication right away. Just as importantly, make sure your estate agent fully understands the treatment and is able to explain it confidently to a buyer.
Good luck! Hopefully you will get it managed and under control without too much hassle.
