Four decades after it began as a small storefront mental health clinic, the Lynn Community Health Center has evolved into the city's largest outpatient healthcare provider, serving about 34,000 patients a year. Now the nonprofit is in the midst of a major building expansion that the agency says will provide the added space needed to meet the growing demand for its services. The $18.8 million project consists of a 55,000-square-foot addition to the center's existing 30,000-square-foot main building at 269 Union St. "We are really excited about being able to provide excellent services to more patients that desperately need care," said Lori Berry, the center's executive director. "For primary care and for all our services, the demand is more than we can handle." According to Berry, the expansion will allow the organization to serve at least 6,000 additional patients. Work is nearly complete on the project's $11.9 million first phase. Begun in July 2010, that work involves construction of the three-story addition and development of its first floor to house a new urgent-care center that will replace a smaller walk-in clinic.