A’s Try Out A Treehouse

The Treehouse, which has room for 3,500 people, will open for the coming baseball season at Oakland-Alameda County Coliseum. (Courtesy Oakland Athletics)

Major League Baseball's Oakland Athletics are using Oakland-Alameda County Coliseum as a test site for their new ballpark project. The Treehouse, a new social gathering space above the left-field bleachers at the multisport venue, is one more example of rethinking the baseball experience.

The 10,000-square-foot retrofit, announce...