Japanese knotweed saves £11.8billion off property values

Is Japanese knotweed really shaving billions of pounds off house prices? Around 4% of UK homes are affected by the invasive plant, study claims

  • As many as 4% of British homes are affected by Japanese knotweed
  • The invasive plant makes a property significantly more difficult to sell 
  • Buyers will struggle to secure a mortgage on a property where it is found

Japanese knotweed is responsible for shaving £11.8billion off the value of Britain’s property market, new research by a removal specialist claims.

As many as 4 per cent of British homes are affected by the invasive plant – either on the property itself or on a neighbouring property.

The invasive plant makes homes significantly more difficult to sell as buyers can struggle to secure a mortgage on a property where it is found.

However, Britain’s biggest mortgage lenders told us that is possible to get a mortgage for a home affected by knotweed, but conditions may be imposed.

Japanese knotweed is an invasive plant that makes a property significantly more difficult to sell as buyers

Japanese knotweed on a property reduces its value by an average of 5 per cent, according to the figures from removal specialist Environet.

It used that to estimate that with 890,000 households across the county are being hit by a typical reduction of value of £13,200 due to knotweed, this equated to £11.8billion in total.

The plant can be stopped from spreading – although this process can be costly, at around £2,500 for a 10sq m area for a herbicide treatment or £5,000 for a 10 sq m for an excavation.

Environet claims that removing the root system from the ground is the only way to deal with Japanese knotweed decisively with minimal change of regrowth.

It said that despite the lower costs, herbicide treatment is increasingly recognised as a control method only. 

This is because above-ground growth can disappear, but the root system beneath the ground is often induced into dormancy meaning it’s capable of regrowing in the future – particularly if the ground is disturbed by landscaping or building work. 

Environet says removing the root system from the ground is the only way to deal with Japanese knotweed decisively with minimal change of regrowth

Nic Seal, of Environet, said: ‘Those buying and selling property are legally required to declare if the property is or has been affected by Japanese knotweed, but if an infestation has been professionally excavated with an insurance-backed guarantee to satisfy mortgage lenders, it is possible to restore the property value to close to the original value.’

He added: ‘Herbicide treatment of knotweed has always been very popular due to the lower costs, but the message is getting through that it’s only a control method and won’t solve the problem definitively.

‘Buyers are much more wary of buying a property which still has knotweed rhizome beneath the ground as there’s no way of knowing whether it’s completely dead. There’s also an environmental cost to using chemicals, which is of growing concern.’

Environet explained that the excavation element can be carried out during the winter months, allowing for full use of gardens during the summer.

What mortgage lenders say about knotweed 

Mortgage broker SPF Private Clients, explained that those buying a property where Japanese knotweed is found may find it less of a deal breaker than in the past where the lender may have automatically declined a mortgage application.

SPF Private Clients’ Mark Harris, said: ‘Should Japanese knotweed be identified, there are four categorisations assessing its severity, with 1 being best-case scenario and 4 being worst-case. 

‘Depending on which silo the property falls into, and whether there is specialist eradication work either ongoing or planned, and insurance in place, lenders may be willing to consider the application.

‘Depending on the severity of the problem, lenders may tailor the amount they are prepared to lend, or not lend at all.’ 

While securing a mortgage on a property with knotweed can remain challenging, lenders confirmed that they are open to providing finance if a management plan is in place. 

A Nationwide Building Society spokesman said: ‘Our policy on Japanese Knotweed depends on how far the plant is from the property. If it is less than seven metres away from the property, we would request a specialist report about eradicating it before deciding whether we could lend. 

‘If the plant is more than seven metres away, we would need written confirmation from the borrower that they want to proceed with their mortgage application despite the presence of the plant. 

‘What may be required is assessed on a case by case basis. Where the valuer identifies the presence of Japanese Knotweed, they may advise that a specialist report is required with respect to eradicating the plant and, where applicable, to report on repairing the property. Any report for eradication of the plant should include an insurance-backed 5 year warranty against re-infestation.’ 

And spokesperson for Halifax explained: ‘The presence of Japanese Knotweed itself is not a barrier to lending. 

‘We will be guided by the surveyor’s, and any subsequent expert’s, report on the scale, location and effects of any presence on or around the property.’  

How were the figures calculated? 

Official figures from the ONS show there at 27.8million households in Britain.

Environet disregarded 20 per cent of households that are flats as these are less likely to be affected by knotweed. 

That produces a figure of 22,420,000 homes in Britain. 

Environet’s survey conducted with YouGov in 2021 revealed that around 4 per cent of homes are affected by knotweed, either directly – meaning that it grows on the property – or indirectly where a neighbouring property is affected. 

It means 889,600 homes are affected in total, according to Environet.

The average value of a property in Britain is £264,244, according to Land Registry’s figures for August. 

Environet claimed that Japanese knotweed reduces the value of a property by 5 per cent on average. This is based on its own anecdotal evidence of what a property is worth once a knotweed management plan is in place (ie the 5 per cent reflects the amount that a buyer might try to reduce an asking price by due to the stigma and risk of the knotweed returning after treatment or removal). 

The 5 per cent reduction translates into £13,212 being knocked off the average home.

As such, the total amount knocked off property values in Britain as a result of Japanese knotweed is therefore 889,600 households multiplied by £13,212, which is £11,753,395,200.


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