Under the plans expected to cost £34m, the interior of the present building completed at the start of World War II, will ripped out to leave only the building facade.
Construction work is hoped to start in January and will entail adding an extra storey, along with an atrium and a new council chamber.
The council has justified the scheme warning that it faces a bill of £14m to complete the maintenance backlog on the existing offices.
The newly-created council building will be more energy efficient and will see the building’s capacity rise from 500 to 1900 people.
Under the proposals, the St Mary’s Gate and Roman House sites will be sold in 2013/14 to generate about £4.6m, while the rent paid for other council buildings will be reduced.
Interested firms have until 23 December to return PQQs. Further information is available from Mace Group, which is advising on the project. Email: Elliot.Sellars@macegroup.com