Fairfax County will petition the Virginia legislature this winter to raise the legal drinking age to 21 and to allow the county to expand further into the road-building business, officials said yesterday.

County Executive J. Hamilton Lambert and four county supervisors presented a preliminary wish list to state Sen. Richard L. Saslaw (D-Fairfax) and three delegates from Fairfax yesterday, renewing a fall ritual that precedes the General Assembly's annual session. The county is asking for less than usual this year, responding to legislators' complaints that Fairfax, with 596,000 residents the most populous jurisdiction in the state, has made too many demands in the past.

Although the full county Board of Supervisors has not officially ratified the legislative package, supervisors at yesterday's breakfast meeting said the county's top priority will be to persuade the state to lift the annual limit on county road expenditures. Fairfax officials, frustrated by slow responses from the financially strapped State Highway Department, persuaded the General Assembly two years ago to permit the county to improve its own roads, but the state limited Fairfax's expenditures to $10 million a year.

Since then, Fairfax has won permission from voters--themselves frustrated by the county's antiquated and overcrowded highway network--to sell $55 million in bonds for road improvements. The $10 million annual cap, however, has put a brake on the county's program.

Democratic Del. Vivian Watts and Republican Del. Warren E. Barry said yesterday they believe the General Assembly may agree to expand Fairfax's powers, although no other jurisdiction in the state has control over its roads. "They don't have any problem with us taking more of our money to accomplish what they ought to be doing," Barry said with a laugh.

Barry also said he will introduce legislation to return the drinking age to 21, where it stood until 1974. Currently 19-year-olds may buy beer and wine without restriction, and 18-year-olds may order beer or wine at bars and restaurants.

Concern about drunk driving, particularly by teen-agers, has spurred the movement to raise the drinking age, but Barry said he is unsure a bill can get through during this term's short, 46-day session. He said he might propose a study of the issue, and Saslaw suggested a bill to raise the legal drinking age to 21 in one-year stages.

The county also will ask the General Assembly to tighten drunk driving laws by recognizing a certain level of alcohol in the blood as automatic proof of drunkenness. Watts said she would introduce such a bill. Currently prosecutors may use blood tests only as evidence that may be rebutted in other ways.