Plans to decarbonise Uttlesford Council's offices have come under fire from the Liberal Democrat group.
The council will spend £741,373 on the project, which was described by Lib Dem group leader Geoffrey Sell as a "colossal white elephant" and "a scandalous waste of public money".
Mr Sell criticised the administration's decision to invest a total of more than £1 million to decarbonise offices in a building which may no longer be required following local government reorganisation.
The council secured £626,299 in government funding for the project but must provide matched funding of £741,373 - much higher than the originally expected £90,861.
The additional cost will be covered by borrowing, resulting in a total estimated expenditure of £1,027,901 over 15 years, including £286,528 in interest payments.
Richard Silcock, deputy leader of the Liberal Democrat group, said the party supports decarbonisation and reducing the council's carbon emissions in principle.
He said: "However, this should not be done at any price.
"This scheme does not represent best value for money.
"It is committing public money on a building whose future is very uncertain.
"Uttlesford District Council will cease to exist in April 2028 and the building will likely become superfluous to the new unitary council's requirements."
Mr Sell urged the council to return the government grant and cancel the project.
Neil Reeve, councillor responsible for environment and climate change, said the project will help deliver the council's net zero ambitions.
He said: "The council has a duty to respond to the climate emergency and to demonstrate leadership by showing that decarbonisation is achievable, even in challenging heritage contexts.
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"Although the overall cost of the project is higher than originally anticipated, this reflects both the technical complexity involved in retrofitting a historic, listed building and a significant increase in the cost of materials in the last 18 months.
"While the future structure of local government in Uttlesford remains uncertain, it is anticipated that there will continue to be a requirement for a council presence within the former Uttlesford district."
Parts of the council building are also let to rent-paying tenants on ongoing leases.
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