A 150-by-30-foot section of stucco concrete fell from the underside of a balcony outside the new Prince George's County Administration Building in Upper Marlboro early yesterday, crashing about 15 feet to the ground.

No one was injured in the incident, which occurred shortly after 2 a.m. In the daytime the area under the balcony is frequently used by perosons going to the building's cafeteria and in good weather it is used as a patio by people eating lunch.

Wooden supports were installed beneath the balcony to ensure that no further naterial would fall, and builders and architects associated with the building expressed worry about the stability of other parts of the two-year-old structure.

We will have to go through the entire building and make certain there are no other areas where this could happen," said county chief building inspector," said county chief building inspector Arthur W. Brown. "But right now we're still trying to deternimine just what caused this."

Brown said his initial inspection indicted that wires that held the heavy stucco concrete suspended beneath the balcony may have come loose. He said there was no evidence that the recent rains had anything to do with the icident. He said that if only the stucco -- in effect a ceiling -- has to be replaced, "a cost of about $35,000 would not be a bad held to that becessarily."

"This is a real jolt," said Clyde Grimm, architect of the six-story, $7.7 million structure that houses main offices of the county government and dwarfs other atructures in Upper Marlboro. "It will take us a while to figure out what happened. But the first thing we have to do is make sure it can't happen again."

William Barton, projects director for Harkins Builders, which built the office structure, said essentially the same thing. He said his company will contact the subcontractor that put the ceiling in -- Chesapeake Applicators of Baltimore -- to discuss the problem.

Barton and Grimm each brought their own men along to examine the ceiling.

Looking on was Assistant County Attorney John R. Barr, whose office will eventually have to decide who -- if anyone -- the county should hold liable for the accident.

Brown said it would be "at least two or three months" before new material could be put in place and several days before an exact cause of the accident could be determined.