Work will start next autumn on the 160-bed four-star hotel with rooftop fine dining restaurant, spa and wedding and conference facilities.
The scheme will be submitted for detailed planning over the next few weeks.
The historic Guildhall building will house the hotel, while more modern extensions at the back of the building would be demolished and rebuilt.
The remaining part of the site, including the old fire and police stations, are subject to a mixed-use redevelopment which will be revealed soon.
John Wilby, project lead for Ascena, said: “We are delighted that this scheme has taken a significant step forward.
“We will soon be submitting the development for planning and it is likely that all being well, that work will start on site in autumn 2020.
“It is a landmark development for the city and a key destination for people across the East Midlands. The Guildhall is an amazing building and we are naturally delighted to be working the city council.”
There will be sensitive restoration of the Grade II listed Nottingham Guildhall – the city’s former magistrates’ court – and Fire Station House, he added.
“There will be a new lightweight extension to the northern elevation of the existing Guildhall to replace an unsightly latter-day extension,” Wilby said.
“We will be utilising high-grade materials and are working closely with our heritage consultant Turley and Historic England to ensure that this listed building is brought back to its former glory.”