Another South Lake Union office project is changing hands.

Los Angeles-based Kilroy Realty said in a regulatory filing this week that it has agreed to buy an unnamed 319,000-square-foot complex — which insiders identified as the Westlake/Terry Building — for $170 million.

Owner Vulcan Real Estate and Group Health put the full-block complex up for sale in August. The office space is occupied by Group Health’s headquarters and Microsoft.

The sale is expected to close by the end of March, Kilroy said. The property is between Thomas and Harrison streets, Westlake Avenue North and Terry Avenue North.

The tentative deal comes weeks after Vulcan sold the nearby 11-building headquarters complex it built for Amazon.com to the giant online retailer for $1.15 billion.

The Westlake/Terry Building’s sale price breaks down to about $533 per square foot, less than the record $644 the Amazon complex fetched.

Kilroy has been on a buying binge in the Seattle area. Since late 2010 the company has acquired six office properties totaling 1.7 million square feet in Redmond, Kirkland, downtown Bellevue and Seattle’s Fremont neighborhood.

Eric Pryne: epryne@seattletimes.com or 206-464-2231