Across the street from Rohnert Park’s Green Music Center, construction workers are transforming a wide tract of land into the largest homebuilding site for Sonoma County in years. In the coming months, three separate housing companies are slated to construct a total of 175 single-family homes on what was once farmland off Rohnert Park Expressway. The three projects, the first to be built in the city in 15 years, comprise the start of a new University District neighborhood that one day could hold more than 1,600 housing units.