A council has been warned revamping a historically significant building could cost five times its £1m estimate.

A former Faversham town and Swale borough councillor fired the warning shot after members unveiled plans to acquire the creekside Town Warehouse, amid fears the Grade II*-listed building continues to deteriorate.

Liberal Democrat members believe its chances of survival and having a future for the community are much stronger in town council hands and is asking Swale Borough Council (SBC) to hand over possession.

But ex-councillor Mike Cosgrove, who was involved in a number of conservation projects while on SBC’s regeneration committee, warns the town council’s estimate of almost £1 million to conserve the building could be a serious underestimate.

Existing Labour members on the town council are also expressing concerns about the potential future costs and a perceived lack of a business plan for the 15th-century building, which has historically been known as TS Hazard.

But the town council has agreed to approach SBC to seek a transfer of ownership of the medieval building, along with the surrounding area of the Town Wharf and pump station.

A report to the town council says the conservation of Town Wharf and its use for educational, recreational and leisure purposes is essential to the future mixed regeneration of the area and the attraction of visitors.

It is hoped TS Hazard could, in the future, accommodate a cafe and be a venue for events and heritage displays to generate income.

The council estimates its restoration could cost £1 million, but Mr Cosgrove fears the realistic potential cost could even be “north of £5 million”.

“This is a building of unique national importance and it requires national expertise to fully investigate what is needed to preserve it,” he said.

“As the building is prone to flooding in winter, it also really needs raising up a couple of feet, which is doable but would cost several million alone.”

Labour members on the town council have also expressed concern about the financial liability the council would be taking on.

They put forward a motion asking the authority to submit an “expression of interest” in taking on the building and prepare a fully detailed business plan before committing to any transfer. But Liberal Democrat members rejected it.

Speaking after the meeting, Cllr Rob Crayford (Lab) said he fully appreciates how historically important TS Hazard is to the town.

But he fears the financial risk to the council has not been fully investigated and assumptions of forthcoming grants are “bullish”.

“We should have a robust business plan before we commit to what could be a massive debt to minimise the risk to the town council and council taxpayers,” he said.

Current town mayor Cllr Trevor Martin believes the council’s own estimate of almost £1 million to fund its restoration is at the “top end”.

He remains confident that most, if not all, of the funds will come from successful grant applications over the forthcoming years, with the aim to open the building to the public by 2028.

“The town warehouse is a huge historical feature of the town and, together with the surrounding quay, could be a very important community asset,” says Cllr Martin

“It has not been cared for as well as it should have over the years. It would be cultural vandalism if we allowed it to continue to deteriorate and I think most people in the town would agree.”

The initial aim, he says, is to make TS Hazard watertight, safe and secure and then engage structural engineers to assess and produce a programme of works.

“The fact is that the town council is in a much better position than Swale to apply for grants and we have plenty of heritage experts in Faversham who know where to help us find the funding. And we already have a grant of £50,000 to start the assessment work.”

But the “worse case scenario” he says, if sufficient money is not forthcoming, is that the council takes out a Public Works Loan Board loan to make up the shortfall.