The deal triggers the first stage of the major Brent regeneration project known as Northwick Park.
Around 50% will be affordable tenures with the rest for private sale or rent, with proceeds reinvested in building more affordable homes for people in housing need.
The overall regeneration of the Northwick Park former hospital site will eventually provide 1,600 new homes, delivered over multiple phases.
The big scheme will also include student facilities, commercial space, and a replacement nursery across 19 buildings on the site.
The full transformation of Northwick Park is being delivered through a partnership between Network Homes, London North West Healthcare NHS Trust, Brent Council and the University of Westminster.
David Gooch, Network Homes Executive Director for Development, said: “I’m really pleased we’ve exchanged contracts with Countryside to get this fantastic project underway.”