The Fairfax County Board of Supervisors, citing worsening juvenile crime statistics in the county, yesterday unanimously approved expanded plans for a new, $3 million youth detention facility.

The board first okayed blue prints for a 20-bed juvenile center last August, but revised the number upward to 33 beds yesterday after reviewing statistics on juvenile detention at the suggestion of a citizen advisory panel.

"Our problems are increasing, not decreasing," said Supervisor Joseph Alexander (D-Lee).

The revised plan approved yesterday would permit further expansion to 55 beds in the future. The new facility, to be located on Page Avenue in Fairfax City, is scheduled for completion in spring 1982.

The board's action came after supervisors expressed concern about the county's youth crime rate, which has increased by 8.4 percent in the last year. Juvenile offenses are up 46.8 percent since 1976, according to official figures.

The county logged 16,493 juvenile complaints in 1978, county officials have said.

Vincent Picciano, director of court services for the county's Juvenile and Domestic Relations Court, told the board yesterday that an average of 35 to 38 county youths are detained daily.

"There is no way we can accommodate all of Fairfax County juvenile detention cases" even with the new facility, Picciano said. He said county officials would continue to send some offenders to the 40-bed Northern Virginia Regional Detention Center in Alexandria.

Picciano said the county placed 610 juveniles in detention last year. But because the Northern Virginia regional center was crowded, he said 20 percent of them, or 124, were kept in the Fairfax County jail.

That, in turn, has contributed to crowding at the jail, a two-year-old facility built at cost of $4.6 million.

Court officials say they expect the number of county juveniles placed in detention to increase to 676 next year, 735 in 1990 and 855 by the year 2000. Picciano said yesterday that the average length of stay also has increased, from 11 to 13 days.

The citizen advisory group, which originally recommended a 20-bed facility to Fairfax, asked the board to reconsider after recent revisions in juvenile law failed to lower the number of detentions.

Although truants and runaways are no longer detained under the revised law, Picciano said the same youths often return to the court system charged with other offenses.

Picciano yesterday defended the size of the proposed staff for the new facility after Supervisor Audrey Moore (D-Annandale) said she thought 20 staff members might be too many.

Picciano said the staff would be needed for security purposes and would work different shifts covering 24 hours a day, seven days a week. "We're talking about a facility for people who don't want to be there," he said. "They are not docile people who are happy to be there."

The staff is scheduled to include a physician and a nurse.

County officials expect to split the $3 million cost with the state, and Acting County Manager J. Hamilton Lambert said county and state officials plan to meet next month to discuss funding for the project.