The Virginia House Appropriations Committee today approved an $18.5 billion biennial budget that for the first time fully funds the state's share of the costs of public education.

The committee also pumped more money into social service programs, such as mental health and welfare payments, and approved stepped-up prison construction projects and money for a Chesapeake Bay cleanup and acid rain study.

It approved pay raises averaging 6.58 percent for state employes beginning in July. The legislators also would get a raise beginning in 1988, but not as much as proposed by former governor Charles S. Robb.

Overall, the committee largely stuck with the budget proposed by Robb and subsequent amendments by Gov. Gerald L. Baliles.

The Senate Finance Committee is to release its version of the budget Sunday. The House and the Senate are expected to approve their separate budget proposals without changes on Wednesday and then work out differences before the March 8 adjournment.

The differences "will be nickels and dimes," said Del. Robert E. Harris (R-Fairfax), one of four Northern Virginians on the House panel. Committee members praised Del. Dorothy S. McDiarmid (D-Fairfax), serving as chairman for the first time, for streamlining the budget process.

"We're bringing back our fair share," said Harris, pointing to increases in funding for colleges, state troopers, Metro and commuter rail service in Northern Virginia.

Fairfax County alone would get $59 million more -- an increase of 20 percent -- in state aid to education in the two-year budget that begins July 1.

"The big winner is basic school aid" statewide, said Donald J. Finley, secretary of education.

"The $679 million increase for public education is a 24.4 percent increase in state funding," said Del. Owen B. Pickett (D-Virginia Beach), chairman of the subcommittee on elementary and secondary education. "This major commitment . . . brings us to full funding of the state's share of the Standards of Quality . . . a goal that has escaped us too long."

State spending for education, based on estimates drawn from a complex funding formula, traditionally has fallen 10 to 20 percent short of the target, with localities forced to make up the difference.

The committee endorsed a controversial plan to require most local school systems to prove that they have passed along a 10 percent raise to teachers in the first year of the budget or face a substantial loss of state aid in the second year.

In social services, the committee approved $400,000 for a pilot Food Stamp Work Program to tie benefits to employment, and $3 million for day-care services for low-income working women.

The committee recommended raising from $600 to $1,000 the amount of financial resources a family can have and still receive assistance payments under the Aid to Dependent Children program; expanded a pilot program to pay ADC benefits to families with two parents, and a $100,000 study on teen-age pregnancy.

In mental health, the committee added $8.6 million to hire 157 persons to correct serious staffing problems, including 15 positions at the Northern Virginia Mental Health Institute, and about $8 million for improved services for community-based mental health facilities run by Community Service Boards.

In public safety, the committee approved $350,000 for communication and electronic surveillance equipment for the state Bureau of Criminal Investigation. State police cars ultimately would be replaced after 75,000 miles rather than the current 100,000 miles.

The compensation-retirement subcommittee proposed savings of $40.3 million, the biggest chunk of which would come in the form of a $28 million windfall profit from selling stock held by the Virginia State Retirement System. The sale has been proposed before, but rejected by the Senate and a similar decision is expected this year.

Pay raises for state employes would average 6.58 percent next year, including a 4.57 percent across-the-board raise on July 1. Most employes also would get a 4.57 percent "proficiency" increase on the anniversary of their hiring, and up to 20 percent of the employes of an agency would also get a $1,000 merit bonus, beginning Jan. 1.

The committee rejected as "unwarranted" increasing the legislators' own salaries from $11,000 a year to $18,000, as proposed by Robb. Instead, it backed a smaller increase, to $15,000, plus an increase in office expenses from $250 to $400 a month, and from $9,000 a year to $12,000 the amount available for legislative aides, which could be paid to one or more persons.