Portland’s planning board on Tuesday gave Maine Medical Center the green light to build to a 2,450-space employee parking garage on St. John Street.

The staff garage is part of the hospital’s broader expansion plan, which is expected to cost more than half a billion dollars. The expansion, which is expected to be completed by 2022, would increase the footprint of the hospital’s main campus by about 25 percent and add 19 new operating rooms and 128 single-inpatient rooms.

Construction on the garage at 222 St. John St. is expected to begin this fall. When the new facility is complete, the hospital plans to demolish the current garage at the corner of Gilman and Congress streets, which will in turn be replaced by expanded hospital rooms and patient care areas, along with a modern cardiac unit.

Maine Medical Center expects to file plans for the new medical building next month, the hospital said.