October 2019 Update
Thanks to a risk-assessment framework introduced in 2012, the situation in 2019 has improved slightly, compared with 2011, but the presence of Japanese knotweed will still harm your chances of getting a mortgage (or may adversely affect the rate, or amount of deposit needed). However, it's not impossible, although opinion seems to be that you may have a better chance with smaller, more specialised lenders, than with mainstream institutions.
The RICS Framework
The situation eased slightly in 2012 when the Royal Institute of Chartered Surveyors (RICS) created a framework1 to help lenders assess the risk of knotweed. They defined four categories of risk:
Knotweed is found more than seven metres away in a neighbouring property, or an empty space like a railway bank or wasteland.
Knotweed is within seven metres of your property but not within it.
It is within the boundaries of the property but is within seven metres from a living space. You will need a professional opinion.
The worst. It is within seven metres of the living space and/or causing damage to walls, paths or foundations. This needs immediate professional intervention.
This make it easier for lenders to give a graduated response (instead of a hard yes/no): obviously, the higher the category, the harder it will be to get a mortgage.
(See below for questions raised about this "seven metre" rule).
Mainstream vs. Specialist
Mainstream lenders tend to be the most cautious, but the categorised risk model has helped. The article Are mortgage lenders ‘overly cautious’ about properties with Japanese knotweed? on the Which? website (written May 2019) contains a table showing the responses Which? got when they asked 17 lenders their policies on lending if Japanese knotweed is identified.
Some refuse outright; many will lend on properties in categories 1 or 2; most will either refuse to lend on properties in categories 3 or 4, or only do so if a suitable report and treatment plan (usually with guarantee) is agreed.
The article Japanese Knotweed Mortgage on the Online Mortgage Advisor website (updated October 2019), confirms the reluctance of many lenders, but notes there are exceptions:
As mentioned, many lenders will decline an application where the property has knotweed on or near the boundary.
The good news is that there are some lenders who have a completely different policy to those who are more specialist and approve Japanese knotweed mortgages regularly.
UK approach to Japanese knotweed ‘overly cautious’
The Which? article mentioned above was in response to a recent (May 2019) report from Parliament’s Science and Technology committee2 that calls the UK's current approach ‘overly cautious’. Which? says the report:
... notes that the latest research on the weed released last year suggests the physical damage to property is no greater than that of other plants and trees that are not subject to the same controls.
Currently, any sign of Japanese knotweed nearby can have a ‘chilling’ effect on a property’s sale, the committee found. Yet, mortgage lenders in other countries do not treat the plant with the same degree of caution as we do in the UK.
Noting the reliance on the "seven metre" rule, Which? goes on to say:
However, the parliamentary committee’s report says the origin of the widely used seven-metre rule was a ‘throw-away remark’ in a 1998 research paper.
Indeed, RICS has admitted that the framework is ‘no longer current’, yet it still forms the basis of mortgage decisions.
It is too early to say whether this report will trigger any changes in the attitude of lenders (and the general public) towards knotweed. The Which? table referred to above summarising lenders' policies on lending on knotweed-affected properties also lists whether they would update their policies should RICS issue new guidelines. Virtually all indicate they would at least consider any changes in guidance, without necessarily making a commitment to change their policies. At time of writing (October 2019), RICS do not appear to have made a response to the Science and Technology Committee's report, but as it was only published in May, this is perhaps not surprising.
1 A PDF copy of their report, which defines these categories, can be found on the RICS website at Japanese Knotweed
and residential property.
2 An announcement of the report is on the Parliament.uk website at UK approach to Japanese knotweed ‘overly cautious’, which contains links to its conclusions and recommendations, a summary, and the full report. The inquiry behind it appears to date from 2017 and its status page is Japanese Knotweed and the built environment inquiry.