The project was delayed after the East Midlands Development Agency (EMDA), which was due to provide £5m, was disbanded in 2011.
Derbyshire County Council agreed to give the developer a loan to finish the work which had started in 2012.
It said it had stepped in as the restoration was too important to the local economy to fail.
* Source: Derbyshire County Council
Developer Trevor Osborne confirmed the completion date on Tuesday, and said the loan was "hugely appreciated".
He added that the hotel would attract the visitor numbers Buxton needed to aid local businesses growth.
Council leader Anne Western said: "The Crescent is of national historic significance and its restoration will put Buxton and the north of the county back on the map.
"This project is too important to Buxton and the rest of Derbyshire to fail."
The plans were first put forward to redevelop Buxton Crescent, built between 1780 and 1789, more than 10 years ago.
It was later granted planning permission in September 2010 with funding coming from developers, the Heritage Lottery Fund, English Heritage, Derbyshire County Council and High Peak Borough Council.
The council said it failed four times in its bid to secure a Regional Growth Fund following EMDA's demise.
The exact figure of the multi-million pound loan has yet to be decided but interest will be charged at commercial rates, the authority confirmed.
The first phase of construction, which included repairs to the Pump Room, was completed in August.
Work to transform the historic Buxton Crescent into a £35m luxury hotel and spa is to be completed by 2016.