Fairfax County voters, who last fall rejected an $8.6 million expansion of the three-year-old county jail, will be asked in November to consider a new $20.8 million plan for the correctional facility.

County supervisors ended months of debate over jail overcrowding yesterday with a decision to seek voter approval for two bond issues to improve the jail, even though several supervisors expressed doubt that the new proposals will fare better than the one rejected last year.

One of the two referendums, which will come before voters at the Nov. 7 elections, will seek approval of a 200-cell addition to the existing jail and a new work-release facility at a cost of $16.2 million.

The other asks for $4.6 million to build a 100-person "correctional camp" at which prisoners would be given rehabilitation training in various skilled trades. Both proposals were produced by a county law enforcement taks force that included Sheriff M. Wayne Huggins, who runs the jail, and Commonwealth's Attorney Robert F. Horan, the county prosecutor.

Huggins applauded the board's decision yesterday, saying that over crowding in the jail had contributed to an incident Sunday night in which two inmates smashed open a second story window and escaped. Both were captured in Fairfax City within hours.

The Fairfax County jail, which has an official capacity of 198 prisoners, has been overcrowded ever since it opened in 1977. Last year's average jail population was 260, and Huggins said it has reached 400 this year.

He said he was optimistic that new data on overcrowding would make voters more favorably inclined toward the issues than they were last year, when they rejected a proposal to add 134 new jail cells and another 30 work-release beds.

But huggins was clearly worried about the chances for the prison camp, which has not yet been designed or sited. "Due to a lack of understanding on the part of citizens, there is a good chance that will get voted down," he said.

That concern was shared by members of the board, who voted to sever the prison farm question from the other proposal for fear it could trigger the defeat of the entire package. "Our primary need is for the expansion of the jail," said Supervisor Nancy Falck (R-Dranesville.) "At this point [the prison camp] could be absolutely the final straw that broke the camel's back."

The supervisors also agreed to create nine new positions in the Sheriff's Department, at a cost of $219,000.