Berwick Preservation Trust has revealed it has a 50 year lease arrangement with the Youth Hostel Association which will cover the entire cost of running the building.
Ian Douglas of Berwick Preservation Trust, speaking at Northumberland County Council's Berwick area committee, said: "We cannot see any situation where we will have to come back for ongoing running costs."
Around £2.8 million has already been secured towards the ambitious scheme but with an estimated cost of £4.5 million there is still a £1.7 million funding gap.
Berwick Borough Council has agreed to try and secure a further £300,000 as a one-off contribution although concerns have been raised by some members that the building could be a financial lame duck.
The Grade II listed building was immortalised in a 1936 drawing by L S Lowry but has been disused since the mid-1980s.
The Trust wants to create a multi-use facilitiy which provides a 42-bed youth hostel, first class exhibition rooms, meetings rooms and a ground floor bistro/outdoor cafe.
It aims to lease back the second floor of the building beside Bridge Street Car Park which will then be operated by a charitable organisation.
The Trust's Executive Officer John Smithson revealed they plan to make a case to Northumberland County Council for a financial contribution towards the cost of the project.
"This is the last of Berwick's granaries that has not been developed," he said. "It is part of the town's economic history and this project represents the last chance for the building."
He added: "If we do not get the funding and it fails what will happen to the building which is listed? It is owned by Berwick Borough Council who would then be left with a derelict property and they are already putting in about £10,000 a year to maintain it as it is. The future for the building and the owners if it does not go ahead would be pretty grim."
However, he stressed: "Everything we have undertaken so far we have carried through to a satisfactory conclusion and we do not aim to fail now."
The Trust is making several bids, including an £800,000 appeal to the Big Lottery Fund and £525,000 to the Northern Rock Foundation.
Mr Smithson said a special steel frame needs to be put into the granary which is likely to cost in the region of £1 million and he revealed that English Heritage have agreed to a special walkway being created to link the Quay Walls with the main exhibition area of the facility.
Councillors were told there are plans to provide the only Museums, Libraries and Archives Grade l centre between Newcastle and Edinburgh and there are hopes that it will lead to top class exhibitions being staged at Berwick. The Trust, which is continuing to raise funds for the refurbishment, have already completed ten projects in the town.