Loudoun County’s only hospital is opening the first phase of a $39.2 million renovation and expansion of its facility on Cornwall Street in Leesburg this week.

The hospital, part of the Inova Health System, will open its new emergency department at the Cornwall Street campus Thursday, along with a new diagnostic imaging unit and an expanded lab. Renovated facilities for behavioral health, child advocacy and the Loudoun Free Clinic will open at the site late next month, hospital officials said.

H. Patrick Walters, chief executive of Inova Loudoun Hospital, said the new emergency department is a “dramatic upgrade” over the old emergency room at that location. The new emergency department is 34 percent larger than the old one, with 14 private exam rooms, four rooms designed for mental-health patients and one gynecology room, hospital officials said.

The renovated facility also has expanded triage space, a two-bed trauma room, a new CT scanner, a medication preparation room and outdoor mass decontamination showers that could be used in a catastrophic event, officials said.

The Leesburg facility opened in 1974 but was vacated in 1997 when the hospital moved to the Lansdowne campus to be closer to the center of Loudoun’s rapidly growing population. Since 1997, portions of the building were leased to other organizations, including county agencies and Shenandoah University. The hospital reopened an emergency department in the building in 2003, two years before the merger with Inova.

Inova has been planning the renovation and expansion of the facility since the merger, Walters said. He said the central Leesburg location makes it the closest hospital facility for two-thirds of the geography of the county.

“This is a tremendous asset for the western part of the county,” Walters said.

The renovation began in January 2012. The building was gutted to its concrete floors and steel, project manager Deb Crosley said, and a new brick facade was added. The new emergency department is on the opposite side of the building from the previous one.

The next phase of the project will include the renovation of the 22-bed Loudoun Adult Medical Psychiatric Services and the spaces that house the Loudoun Free Clinic and Loudoun Child Advocacy Center. After those facilities open late next month, some older buildings on the campus will be demolished to create parking space, Walters said.

Loudoun Adult Medical Psychiatric Services will serve the entire county, said Edward Puccio, medical director of the emergency department, including patients who are first admitted to the Lansdowne campus and need psychiatric services.

“We are very fortunate to have a mental health unit at our hospital,” he said. “Hospitals that don’t have that struggle with getting mental health patients admitted, and our process is very efficient.”

Walters said the renovation has been enhanced by donations. The hospital is about halfway to meeting its goal of $8.5 million in donations for the project, he said.

The donations have “helped us exceed expectations for this campus,” Walters said.