Galliford Try was appointed as the council’s preferred bidder for the scheme in July 2017.
Galliford Try Partnerships & Regeneration and Ealing Borough Council have now formalised a deal worth an estimated £275m for the Perceval House project.
This project involves the construction of 470 new homes alongside a new council headquarters building, a library and customer services centre, retail premises and residential and commercial parking areas.
Work on site is not expected to start this year.
Galliford Try chief executive Peter Truscott said: “Our Partnerships business continues to build a strong track record in major regeneration schemes within the capital and we look forward to delivering high-quality homes and facilities for the council and the people of the borough."
In a separate but related initiative, the old Town Hall is being redeveloped into a hotel by Mastcraft, for which a planning applilcation was submitted at the end of January.