This depends on your mortgage lender, however, there have been some cases where lenders have refused to remortgage properties because there is a serious Japanese knotweed infestation on the neighbouring property. Whilst the government is currently looking into ways to soften the impact that a Japanese knotweed infestation has on home values, many mortgage lenders still operate a hands-off policy. Hopefully your neighbour realises the effects and potential litigation that could come their way if it is left untreated and they secure a professional treatment plan which comes with an Insurance Backed Guarantee.