When complete, the project will see 2,000 county council staff moved out of 12 buildings in Northampton, including County Hall, into one structure.
Work started on the authority’s former staff car park in Northampton’s Angel Street in February last year.
The topping out ceremony, set for Tuesday, May 3, signals that the building has reached its “highest point,” a county council spokesperson, said.
Leader of Northamptonshire County Council, Councillor Jim Harker, (Con, Ise) said: “I’m delighted that we’ve reached this very significant stage of the build and it’s exciting to see the structure rise up in the heart of Northampton.
“Not only is this an important building for the county council but it is also part of the major regeneration that is taking place in the town.
“By bringing workers into the town centre there will also be a big boost to the local economy.”
The work is being carried out by contractor Galliford Try and is expected that the building will be formally handed over to the county council in November.
Staff will begin moving in in January 2017.
It is proposed that the retained parts of County Hall will be maintained by the county council for its democratic function – namely the council chamber, the councillors – and a small number of staff.