Sheffield's Old Town Hall will cost more to restore than the £10 million initially forecast, say campaigners, who remain 'cautiously optimistic' it can be saved.
The city's former administrative HQ in Castlegate, which also housed Sheffield's courts, has lain derelict for more 20 years, during which time the Grade II-listed building has gradually deteriorated.
The Friends of the Old Town Hall is trying to raise the money to buy the building and bring it back into use, which it estimated earlier this year would cost more than £10m.
It says the anticipated cost has now increased, based on an outline business plan commissioned by the group, though it has refused to reveal what the new expected price tag is at this stage.
But the group's chairwoman Valerie Bayliss remains confident the mission is achievable and says the report makes her even more convinced that a commercial sale of the property, which has been on the market for around two years, is not viable.
"We now have a very detailed costing of the work that needs to be done and it's going to be more than the £10m we had forecast, but we're cautiously optimistic it can be done," she said.
"Make no mistake, this is going to be a very long job, and raising money on the scale required is going to take a long time, but we're determined to get on with it.
"But the architects and surveyors have reinforced our view that the building's structurally sound and although there's a lot of damage inside it can be fixed.
"I can't see a commercial solution for the building. A private developer would have to put in too much of his or her own money to ever get a return on that investment. We believe grant funding is the only answer."
The group's vision is to make the historic building a community-owned asset run by a not-for-profit organisation, which would generate the income needed for ongoing maintenance.
The restoration would take place in three phases, with a variety of uses envisaged for different spaces within the venue.
Those include work space for creative and digital firms, meeting rooms, performance and rehearsal areas, and cafés, restaurants or bars.
"There are some wonderful spaces in there which if done up could look very attractive, and we hope to be in a position to go public with our plans in the new year," said Mrs Bayliss.
She was speaking after a litter-pick organised by the group in Castlegate on Saturday, and funded through a council grant, during which around a dozen volunteers between them collected 12 bags of waste.
She said littering in the area had reduced since the group carried out a previous clean-up in April, and she hoped the latest efforts would further discourage people from dropping rubbish.
The Old Town Hall has been owned since 2004 by G1 London Properties Ltd.
The group is pressing Sheffield Council to fix the roof of the building, which campaigners say is already badly water-logged, after the council announced this summer money was available for urgent repairs.
The friends group received a total of £12,500 from the Heritage Lottery Fund and the Architectural Heritage Fund to commission an investigation into the business case for restoring the building, which was carried out by Integreat Plus.