BANGOR, Maine — The first time Michael Roberts walked into his new office, it felt incredibly familiar.

The space the deputy district attorney for Penobscot County will occupy on the second floor of the historic courthouse on Hammond Street once belonged to his father. David Roberts worked in the same corner of the courthouse when he was a Superior Court justice from 1967 to 1980. The elder Roberts died suddenly in January 1999 at the age of 70 after nearly 20 years on the Maine Supreme Judicial Court.

“I was 12 when he first went on the bench,” Roberts said Wednesday. “I remember visiting him in that office.”

The desk that has been moved into his office, which overlooks Hammond and Court streets, also may have been used by Justice Roberts.

“No one is really sure if it was his desk,” the prosecutor said, “but it would be nice if it was.”

Roberts and his co-workers in the Penobscot County District Attorney’s Office have spent this week moving from office space in the annex behind the nearly 100-year-old building and the courthouse basement into renovated space once occupied by the Superior Court.

The space was vacated in late 2009 when the Penobscot Judicial Center opened, combining the District and Superior courts in one building. The move is expected to be completed this week, according to Penobscot County District Attorney R. Christopher Almy.

The renovated second floor will nearly double the square footage for the DA’s office from 3,000 square feet — 2,000 in the annex and 1,000 in the courthouse basement — to 5,000, Tom Duffy, facilities manager for the county, said Wednesday.

Prosecutors’ offices are larger and individuals who work on similar cases will be in the same areas, according to Almy. The office of the Victim Witness Advocate, which was in the basement of the courthouse along with an office out of which three prosecutors worked, also will be located on the second floor.

“We’ve added security,” Almy said, “and that should help with morale.”

A reception area will help keep visitors from wandering into working areas as they could in the old space. In addition, there is a kitchenette, conference room and storage space on the second floor.

Almy also said that his former office, the largest in the annex, often had to be used as a conference room so prosecutors could meet privately with witnesses.

“We’d have preferred to be in the [Penobscot] Judicial Center, but the judiciary did not see the merit in that,” Almy said Wednesday in his old office in the annex. “The commissioners saw our need for additional space.”

The downtown lot between Exchange Street and the Kenduskeag Stream on which the Penobscot Judicial Center was built is in a 100-year flood plain. Because the stream occasionally overflows its banks, the building could not have a basement. That essentially eliminated one floor and the DA’s offices from the design.

As the judicial center was being built, county commissioners sought a longtime tenant for the former District Court building on Hammond Street to replace income lost when the courts consolidated. It is now occupied by the U.S. Postal Service and the Red Cross.

Once modifications to that building were completed last fall, work on renovations to the historic courthouse began in November, Duffy said Wednesday. In addition to the DA’s offices, the office space occupied by the Registry of Deeds and the county’s administrative staff were remodeled.

All but about $50,000 to pay for the renovations came from a $475,800 grant from the U.S. Department of Energy in stimulus funds, County Administrator Bill Collins said.

New energy-efficient windows, lights, floors, carpeting and central air conditioning have been installed throughout the building, Duffy said Wednesday.

“We put in two to three inches of foam insulation on the outside walls,” he said. “Now we’re probably at an R [rating of] 18 to 20. Before, we were an R [rating of] 10 if we were lucky. It was leaking like a sieve.”

While Duffy supervised workers, county commissioners oversaw every aspect of the work.

“We, the commissioners, think we have a wonderful historic building,” Commissioner Peter Baldacci of Bangor said Wednesday. “There have been no major renovations here in almost 100 years.

“We want this building to be vital for another hundred years,” he continued, “so we’ve made it as functional and energy-efficient as we could and provided the DA’s office with very good workspace.”

Once the move by the DA’s office has been completed, the Penobscot County Sheriff’s Office will move out of the jail and into the annex, according to Collins. The second-floor courtroom will be used as the commissioners’ meeting room.

Major changes to the third floor, where the Penobscot Regional Communications Center is located, are not planned for the immediate future, he said. The third-floor courtroom is being used for training sessions and meetings.