"We anticipated growth. We are just exceeding expectations," Duffy said.
About 100 administrative-staff members will move from the west wing of the hospital's fourth floor, about 70 of them to a temporary modular building on the campus.
Construction of the new rooms is expected to take nine months.
Several years ago, in planning for the new facility that replaced Montgomery Hospital Medical Center in Norristown, Einstein research found that only 40 percent of the population of central Montgomery County used one of the three hospitals located there.
The rest went to hospitals outside the area, which is an attractive market because a high portion of residents have commercial health insurance or are on Medicare, the most lucrative forms of reimbursement for hospitals.
Better reimbursement was a key factor in Einstein's decision to build in Montgomery County. Its 772-bed hospital in North Philadelphia is a major provider of care for beneficiaries of Medicaid, which pays only about 80 percent of costs.
Einstein has changed the competitive landscape in that part of the suburbs, one observer said.
"It's really intense. Einstein is a big part of that intensity," Plymouth Meeting health-care consultant Gerald Katz said of the competition. Katz is also chairman of the board at Chestnut Hill Hospital in Northwest Philadelphia.
Just three miles away from Chestnut Hill, in Lafayette Hill, Main Line Health opened a new multi-specialty office in February.
Temple University Health System opened a facility in Oaks in October 2011 and is scheduled to open a second multi-specialty facility in Fort Washington in the summer.
Contact Harold Brubaker at 215-854-4651 or firstname.lastname@example.org.