REDMOND – Microsoft Corp. plans to spend about $1 billion over the next three years on an expansion that will add enough offices at its headquarters to house about 12,000 workers.

The plan, which calls for increasing Microsoft’s sprawling suburban campus by about one-third, will accommodate both new and existing workers.

The company plans to add another 4,000 to 5,000 workers worldwide, most of them in the Seattle area, during the current fiscal year ending in June.

The company employed about 30,000 workers in the Puget Sound region out of more than 63,000 globally when it released its latest employment figures Sept. 30.

Microsoft, which also is expanding overseas, said its plans call for bulking up its hometown campus more rapidly than previously anticipated.

“What we thought just a year ago we might need to do over the course of a decade or longer, we are now here today to talk about doing much faster,” Brad Smith, senior vice president for legal and corporate affairs, said at a news conference outside a building the company is tearing down to make way for a new one.

The expansion will include seven new buildings, and seven others the company has acquired. In total, that accounts for about half of the development agreement the city of Redmond approved in May, which outlined Microsoft’s development plans for the next 15 to 20 years.

“This reflects our confidence in the future and our commitment to this region,” Smith said.

The company has not yet decided how many of its existing workers will move out of overcrowded offices and join newly hired workers in the offices created by the expansion. More detail on that is expected to be released in the summer, Smith said.

Microsoft said it will spend $35 million over the next three to five years to help improve streets, sewers and other infrastructure, and add an overpass over busy Highway 520, a main thoroughfare between Seattle and its eastern suburbs. The improvements will aim to better deal with the congestion caused by Microsoft’s growing campus.

Microsoft has been gathering up land and real estate in the area over the past few years. In January, it agreed to pay $209.5 million for most of the Redmond office campus of insurance company Safeco Corp.

Associated Press

Brad Smith, senior vice president of Microsoft, details the company’s expansion plans Thursday in Redmond. Behind him are (from left) Gov. Chris Gregoire, Redmond Mayor Rosemarie Ives and King County Executive Ron Sims.