Councillors have agreed on proposals to demolish the Perceval House site in Ealing Broadway.
The site will make way for new homes and an improved customer service centre and library.
Other features include smaller office accommodation for the council.
The proposals aim to 'unlock' the site's potential by redeveloping it for housing, commercial space and replacement council facilities.
Approximately 360 homes could be built on the site and the council could potentially retain a number of properties to provide 'affordable' housing.
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The development could also include some commercial and retail space.
A preferred option is to develop the replacement offices behind Perceval House on a car park, so that council services can transfer to the new site before the old block is levelled.
Alternatively, the council could also move to a different site, either permanently or temporarily, while the building work takes place.
Council Leader Julian Bell said: "We're sitting on top of a potentially very valuable site and when you consider the tough times we're in financially and the desperate need to provide more affordable homes, it makes perfect sense for us to release the site's value.
"Perceval House is now more than 30 years old and many of its electrical and mechanical services will need to be replaced in the near future, requiring major investment. By redeveloping the site we can provide much needed homes, a greatly improved customer service centre and co-located library, as well as smaller replacement offices that are cheaper to run and cater for modern working practices."
The council expects to select a developer to enter into a joint venture in 2016.
Subject to planning permission, the first phase of work could begin in 2018, with the new offices delivered by 2020.
(LM/CD)
