Does Japanese knotweed damage houses?

Japanese knotweed rarely causes structural damage to substantial buildings such as houses, according to RICS, and a 2019 House of Commons committee concluded that its physical damage to property is no greater than that of other disruptive plants and trees. It can, however, damage lighter structures, walls, paths and drains, and because it is so hard to get rid of, it still matters when you sell or mortgage a home.

The RICS position

RICS sets out the surveying profession's position in its standard Japanese knotweed and residential property, which took effect on 23 March 2022. It says knotweed "rarely causes structural damage" to substantial buildings such as houses and is not typically associated with subsidence or heave, but that it can damage lightweight structures, freestanding and retaining walls, paths and hardstandings, and drains. RICS also cites research showing that knotweed cannot grow through intact concrete.

Where a surveyor finds knotweed actually causing visible material damage to a structure, such as a garage, outbuilding, conservatory, path, or retaining or boundary wall, it falls into RICS Management Category A. Knotweed growing where damage already existed, from neglect, disrepair or another cause, does not count.

The research

The study RICS draws on, by Fennell, Wade and Bacon, was published in PeerJ on 25 July 2018. It combined a literature review, surveys of contractors and surveyors, a case study of 68 properties in three streets in northern England, and excavation data from 81 sites.

It found no support for the idea that knotweed causes significant damage to buildings, and certainly no more than other plants. Rhizome rarely extended more than 4 m from the plants above ground, typically staying within 2 m for small stands and 2.5 m for large ones. The authors concluded that the seven-metre rule was not a statistically sound way to estimate rhizome spread, and that automatically restricting mortgages where knotweed is present was "not commensurate with the risk".

The Commons committee report

The House of Commons Science and Technology Committee published Japanese knotweed and the built environment on 16 May 2019. Alongside its conclusion on physical damage, it noted that knotweed is particularly hard to eradicate, needs treatment over several years and can regrow. It also found that the seven-metre rule was being used as a "blunt instrument" in some lending decisions, and that some lenders refused, or heavily restricted, lending on affected properties, findings that predate the current RICS approach.

The committee noted the 2018 study's limitations: its case study used derelict housing built before 1900, and its contractor sample was small. RICS nonetheless drew on it when rewriting its guidance in 2022.

The old 7 metre rule

The 7 m figure was a radius used in the Environment Agency's 2006 Knotweed Code of Practice, which the RICS 2012 information paper adopted. According to the committee, Professor Max Wade traced it to a "throw-away remark" in a 1998 paper. The 2012 paper sorted properties into four risk categories based on a 7 m distance from buildings and boundaries and, as summarised by the Court of Appeal, said roots can extend up to 3 m vertically and 7 m horizontally.

The current RICS approach

The 2022 standard, reissued in October 2022 as a professional standard that RICS members must comply with, replaced the distance-from-buildings test with four Management Categories based on what the knotweed is actually doing.

The only distance in the RICS decision tree is 3 m beyond the property boundary, used for knotweed on neighbouring land; knotweed seen further away is normally only recorded in the valuer's notes. RICS says the 3 m figure was agreed "based on the latest research", drawing on Fennell and colleagues' finding that 3 m is a better measure of typical rhizome spread than 7 m.

Other effects on a property

Gardens and neighbours

RICS treats loss of amenity as a separate issue, covered by Category B. In England and Wales, the Court of Appeal held in Network Rail v Williams (2018) that rhizome spreading onto a neighbour's land can be an actionable nuisance even without physical damage, because it reduces the owner's ability to use and enjoy their land. A fall in market value alone was not enough.

Dense stands also suppress almost all native plants, and stands on riverbanks can hold back floodwater and worsen flooding.

Mortgages

For Categories A and B, RICS anticipates that most lenders will require, subject to a retention from the mortgage, an inspection by a specialist from a recognised trade body and completed works under a management plan with an insurance-backed guarantee. For Categories C and D, no retention should normally be needed. These are RICS expectations, not lender commitments: each lender sets its own mortgage policy, and some do not make retentions at all.

Selling

Even after remediation, RICS says, anyone selling a house with knotweed will have to declare it on the TA6 property information form, the Law Society form used for most standard residential sales in England and Wales. Some stigma may affect resale value, but RICS expects the effect to be much smaller than at purchase and to reduce over time, and tells valuers not to apply a standard percentage reduction for knotweed.

Treatment time

RICS says herbicide control needs at least four years of treatment and monitoring before a completion certificate. Excavation typically extends 2 to 3 m beyond the visible stand and 1 to 2 m deep, so a 2 m by 2 m stand might mean digging out an area 6 m by 6 m, up to 72 m³ of soil. Even sections of rhizome under 1 cm or 1 g can regrow, according to RICS.

Dormant rhizome

Knotweed that has disappeared from view has not necessarily gone. RICS says rhizome can lie dormant in the soil for decades, and the Court of Appeal recorded the Environment Agency's code of practice as saying possibly 20 years. Some herbicide treatments can induce dormancy, and later disturbance, such as digging foundations, can trigger regrowth. This is one reason RICS accepts that a competent inspection can still miss knotweed, along with small stands, visits outside the growing season and growth that has been cut down or cleared.

Next steps

If you are unsure whether the plant is knotweed, see how to identify it. If it is, and it is near a building, wall, path or boundary, a specialist survey can establish how far it extends and set out the treatment options. If you need one, we can introduce you to up to three independent specialists who cover your area; any survey, quote or work is agreed directly between you and the specialist.