Microsoft will buy the Mountain View campus it has leased for about 15 years, and submitted plans last week to overhaul and expand the site to create space for hundreds more workers—and add a green roof.

Microsoft plans to build a state-of-the-art facility (rendering above), EVP Qi Lu wrote in an open letter to employees, according to the San Jose Mercury News.

The existing Silicon Valley campus totals 515k SF, but the finished project would grow that to 644,622 SF and could support up to 3,000 employees. Microsoft has at least 2,000 employees in the Bay Area.

Earlier this month, Facebook announced it had bought its Menlo Park campus. Companies that own their buildings have more control, and several projects underway in Silicon Valley are companies building their own campuses.

As part of the project, Microsoft will return the parking on 6.2 acres on the eastern side of the campus to a natural environment and add a 3.7-acre green roof to the campus. Qi Lu told employees the construction was expected to take at least three years. [SJMN]