Kaiser Permanente has completed the first phase of a large expansion of one of its medical centers in Maryland that when finished next year will make the facility the company’s largest in the Washington region.

The Largo facility, which will double to 233,000 square feet of medical space, now includes an urgent-care center, an ambulatory surgery center and radiology, pharmacy and lab services. The facility began offering the new services last week.

The project is part of Kaiser’s continuing push to expand its presence in the Washington region. Since 2011, Kaiser clinics have opened in the District, Tysons Corner, Gaithersburg and Baltimore.

During the past several years, Kaiser Permanente has invested about $2 billion in the region, said Kim Horn, president of Kaiser Foundation Health Plan of the Mid-Atlantic States. “It’s not only for expansion purposes,” she added. “It’s really to develop a health-care system that prepares for the 21st century.”

Kaiser expects its membership in the Mid-Atlantic to double to 1 million within the next several years, in part as a result of demand driven by the Affordable Care Act, which aims to increase the ranks of the insured, Horn said. “It will allow an increasing number of people to turn to Kaiser, both individuals and small businesses,” she said.

About 17 percent of people under 65 in Prince George’s County, where the Largo facility is located, are uninsured, according to government data. But, analysts said, that is likely to decline significantly as the Affordable Care Act goes into effect.

Kaiser is also tapping into the shift away from hospitals and toward urgent-care clinics. The use of inpatient hospitals declined 10 percent in Maryland between 2006 and 2011, according to a March research report by Kaufman, Hall and Associates, a health-care consulting firm. This trend is here to stay as the market continues to shift away from inpatient hospitals, the report said.

“I think that you are going to find, as we’ve been doing, more services that are ambulatory and outside the walls of hospitals,” Horn said.

An estimated 3 million patients visit urgent-care centers each week, according to the Urgent Care Association of America. Since 2008, the number of these facilities has increased from 8,000 to 9,300. That includes about 400 centers in the District, Maryland and Virginia, excluding smaller walk-in clinics in pharmacies and stores such as Target and Wal-Mart, according to the industry group.

“The shift towards urgent-care centers is becoming more widespread because . . . emergency rooms are so expensive,” said Melissa Randolph, a benefits consultant at Potomac-based Willis of Maryland.