The newly formed Cheshire West & Chester council has had a £28.75m deal approved to sell County Hall to Chester University and relocate to Liberty's HQ development nearby.
Cheshire West & Chester council will move into the entire 82,000 sq ft office element of the new mixed-use building, due for completion in the autumn.
At the start of August, leaked council documents said the 50-year-old County Hall is no longer deemed fit for purpose by the new authority, which is partly owned by Cheshire East council.
Cheshire West & Chester council estimate the move will save council taxpayers £4m across Cheshire over the next five years in running costs and business rates.
Cheshire East council, due a share of the sale proceeds under the terms of local government reorganisation, has now agreed the deal on condition that possible transfer of the authority's interest in the castle square car park be dealt with as a separate transaction and an independent valuation of the car park obtained.
Around 600 to 700 members of Cheshire Wes and Chester council staff will transfer to the HQ office space in spring next year.
The six former borough councils and the county council of Cheshire were reorganised into two new unitary authorities for Cheshire East and Cheshire West & Chester in April.
The local authority in the west occupies 20 offices and wants to reduce this figure by more than half.
As well as offices, HQ will contain apartments, restaurants and an Adobe hotel overlooking the Chester Racecourse.
HQ is being built by Cheshire-based construction firm Pochin. Legat Owen and Savills are joint agents on the offices.
All parties were unavailable to comment.