This equates to 55% of all UK housing stock and accounts for homes that have not been assessed because they have not been sold or rented since the standard was introduced, according to Rightmove.
The firm said energy-efficiency changes needed to be made to these properties to make them greener and to lower energy bills.
Its analysis of Government data showed the average energy bill for a three-bedroom house with an EPC rating of F was £4,431 per year, while a C-rated equivalent home was £1,669 – a difference of £2,762 per year.
Rightmove conducted a study among 14,000 homeowners and renters and found the biggest motivator behind making green changes to their home was reducing energy bills.
It also revealed that 87% of renters and 83% of homeowners feel more should be done to help people make their homes more energy efficient.
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Rightmove has called on the next Government to prioritise green home incentives and make them available to as many homes as possible.
Green support in party manifestos is ‘a start’
Tim Bannister, property expert at Rightmove, said: “While it’s a start to see some green support proposed in the party manifestos, there’s a big job to be done to address the wide-scale issue that at least 18 million UK homes need to be improved.
“Many homeowners and landlords need urgent access to schemes that enable them to make these improvements, which could help with huge savings on energy bills. Each home requires different improvements, and the upfront costs are a huge barrier to change. Without more help, many homeowners and tenants will continue to live in high-carbon-emitting homes with high energy costs.”
The Green Party pledged to invest £29bn over the five years to improve the rating of homes to an EPC B or above in its manifesto for the general election, while the Labour Party announced an investment of £6.6bn to upgrade five million homes.
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