Construction is underway at a new 100,000-square-foot office complex in West Windsor, where law firm Hill Wallack LLP will move its headquarters early next year.

In a ceremony Wednesday, Gottesman Real Estate Partners and Mountain Development Corp. broke ground on the two-building project known as Roszel Square, according to a news release. The first building is scheduled to be complete by early next year, with Hill Wallack set to occupy 43,500 square feet across the top two floors.

Milton Charbonneau, a senior vice president and principal with Cassidy Turley, will market the remaining 19,000 square feet on the first floor, plus the 38,000-square-foot second building in phase two, the news release said.

The new construction, which has become rare in New Jersey’s office sector, is the latest sign of strength for the Princeton submarket, which is among the strongest in the state and a standout among suburban locations. Jones Lang LaSalle tracked its year-end vacancy at 18.8 percent, the third-lowest statewide and well below the 25 percent mark across northern and central New Jersey.