Deputy Fire Chief Joseph Callaghan said the dead man, identified as Guo Tai Chen, 51, of New York City, appeared to have died quickly. Another worker was seriously injured.
"He was trapped under a good amount of rubble," Callaghan said. "He was D.O.A."
The building, which dates from about 1930, houses the Coda Kitchen & Bar, a popular eatery in downtown Maplewood, a leafy suburb outside Newark. Owner Saul Fischer said he wanted to install an inside stair to the basement so his employees would not have to go outside.
Mayor Vic De Luca and other officials said the work had the required permits and the site was inspected as recently as Wednesday.
"It's just an accident. It gave way, and he was there," DeLuca said. "I'm sure he went to work today thinking he was going to go home again."
Callaghan said Maplewood has a lot of "old masonry." In 1999, a woman was killed when a building overhang fell on her.