The council is currently prequalifying firms for the design and build contract with expressions of interest due in by 27 March.
Northamptonshire County Council said the civic building on derelict land in Angel Street would replace 12 existing county council buildings spread across the town.
The new building, which is being outline designed by BDP, will save up to £54m in the future running costs.
The council hopes to sign up a builder in September this year, ready to start work in January 2015 and finish by the end of 2016.
Leader of the council Cllr Jim Harker said: “This is another critical step in our journey in making this ambitious and exciting project a reality.
“This project is all about helping to regenerate Northampton town-centre as part of the Northampton Alive project, while also ensuring our own office accommodation is as efficient and as cost effective as possible.
Contact details: Northamptonshire County Council