The Prince William County Board of Supervisors, buoyed by the unexpected discovery of $2.3 million in additional revenues, voted yesterday to trim the county's property tax rate by 3 cents and purchase bulletproof vests for county police and help build a new senior citizens center near Manassas.

"It was just like Christmas around here," said county Budget Director Blaine Aikin as the board voted unanimously to approve the $107.9 million budget for the fiscal year beginning July l.

The supervisors voted to return half the unexpected revenues to property owners by cutting the real estate tax rate from $1.42 to $1.39 per $100 of assessed value, a move that will cost the county treasury $1.2 million. The owner of a home valued at $100,000 would save $30 a year with the reduction.

It was the second year in a row supervisors have cut property taxes, said Board Chairman Kathleen K. Seefeldt. The board also cut the rate by three cents last year and had promised to make a two-cent cut this year, she said.

The new budget, however, calls for a 16.9 percent increase in fees residents and developers pay the county for services.

Aikin said the county expects to receive an additional $670,000 in revenue from the higher fees, but said $570,000 of that will come from developers for new home inspection services and the processing of building applications. "This hike will not affect the average resident by much," he said.

Aikin said part of the unexpected $2.3 million in revenues came from the county school system, which said it previously had asked for $200,000 more than it needed. The schools account for the largest single county expenditure, receiving $57.7 million of the $107.9 million total. Another $500,000 in new funds came from the county's $4 million contingency fund.

The supervisors voted to spend $110,000 of the additional money to replace several police vehicles and to buy bulletproof vests for 197 officers, said Aikin. They also appropriated $300,000 to build a new senior citizens center, which the county will share with the city of Manassas, he said.

Supervisors also voted to buy $70,000 worth of books for the public library and increased the county's mental health budget by $138,000. "There was a great interest in mental health shown during the public hearings," said Aikin. "The supervisors decided to show their support."

The increase in funding for some county programs comes in the wake of criticism of the budget by public school teachers, who say the 3 percent cost of living raise it provides them is too small.

The county school board asked the supervisors last month to use the unanticipated $1.6 million in property revenues for another 2 percent raise in teachers' salaries, said school spokeswoman Kristy Larson.

"Clearly, the supervisors were not receptive to the idea," she said.