Work to turn the Grade I listed building into shops, cafes, offices and a large town square has been set back by a number of unexpected issues, including the discovery of a graveyard and the subsquent excavation of 200 bodies.
The project, which began in October 2014, had been scheduled for completion this summer.
But council leader Tim Swift said today that the work would take longer than the planned 72 weeks to complete.
Coun Swift said: “It is important to remember that the Grade I listed Piece Hall is over 230 years old. The age and complexity of the building has led to unexpected challenges throughout the project, which the council and our contractor GRAHAM Construction have worked closely together to resolve.
“We have one chance to get this right. It is a considerable investment which demands careful attention to the stone, and a high quality finish to do the building justice.”
The Transformation Project is being funded by the council and a £7m grant from the Heritage Lottery Fund.
The aim is to make Piece Hall a destination in its own right, bringing daytrippers from West Yorkshire, the North of England and far beyond.
A new £9.5m Central Library and Archive, to be built alongside the hall, will be wholly funded by the council.
This will be a striking building with state-of-the-art facilities for the community, supporting the Piece Hall transformation and helping to create an improved pedestrian route from Halifax train station through to the town centre.
Papers due to go before Calderdale Council’s Cabinet on Monday warn that the Piece Hall project is expected to go over budget.
In the report, head of finance Nigel Broadbent said: “As a result of the delays and other necessary changes it is anticipated that there will be a cost overrun on the project and additional resources have been reserved within the capitals programme to fund this.”
A more detailed report will be prepared for the Cabinet seeking approval for additional funding.
In the meantime, the council and the Piece Hall Trust are pressing ahead with plans for the opening, the timeline and the programme of activities.
Coun Swift said: “The Piece Hall is a unique and very special space, and we have a responsibility to restore it both for our own enjoyment but also for generations to come.
“We are confident we will achieve this, creating an amazing heritage, retail and civic space for Halifax and the region to be proud of.”
Graham Construction started work on the site off Blackedge on October 27, 2014.
Since then heritage stonemasons have been delivering a comprehensive programme of conservation repairs to the historic fabric of the 18th century building.
A new restaurant, which will link to the Square Chapel and Piece Hall, was well under way by October.
And in April a Halifax-based heritage roofing expert, Ploughcroft Eco-Roof, completed the restoration of the Piece Hall’s iconic bell tower.
Situated at the Piece Hall’s main Westgate entrance, the bell tower forms a focal point of the famous building, and is visible from far across the town.
The project forms part of Ploughcroft’s wider involvement in the restoration, which has seen it repairing and maintaining the 10,000 stone slates on the building’s sprawling roof over the past 12 months.