At a time when tech companies are competing for office space on the Eastside, Microsoft is doubling down on its 31-year-old, 500-acre corporate headquarters in Redmond.

On Tuesday, the company announced it would spend more than $1 billion to build and renovate 9.2 million square feet of office space on its East Redmond campus. The project is expected to break ground next fall and take five to seven years to complete.

In the end, 12 existing buildings will be demolished, and 18 two- to four-story buildings will be constructed, offering 2.5 million square feet of additional office space. The company also will modernize 6.7 million square feet of office space in its existing buildings. Currently, the campus is home to 125 buildings offering 15 million square feet of office space. When completed, the campus will house 131 buildings, allowing the company to add up to 8,000 more employees.

In addition, the project will include a two-acre plaza large enough for events that can accommodate up to 12,000 people; a new bridge that will span State Route 520, connect the company’s East and West Redmond campuses, and serve pedestrians and cyclists; running/walking trails, sports facilities, and green spaces; company store, retail shops, and restaurants; and an underground parking facility.

Microsoft employs 47,000 people in Washington state. The company moved to its corporate campus in Redmond in February 1986. Its last major expansion was in February 2006, when it created 3.1 million additional square feet of office space.