The Prince William County Board of Supervisors yesterday approved a $101.9 million budget for the coming fiscal year that will hold real estate taxes at their current level.

The board voted a real estate tax rate of $5.25 for each $100 of assessed valuation, the same as the current fiscal year, and, as the county has not reassesed property since 1975, the actual bills paid by homeowners will remain stable. The tax bill on a $50,000 home will remain at $871, for example.

Several other Washington area jurisdictions are planning to maintain or reduce tax rates, but have increased bills by reassessing property upwards. (One exception is Prince George's County, where the county executive has said most homeowners can look forward to lower property tax bills.)

In approving a $12 million increase over fiscal 1978 spending, the Prince William surpervisors set a general spending increase of 4.7 percent for the county school system (which accounts for three-fourths of the budget total) and most county government departments.

The school board's budget request was cut from $79.8 million to $76.1 million. The school board now is considering ways to absorb the reduction, including curts in proposed pay raises, elimination of 38 teaching positions and 26 proposed new positions, and a slowdown in replacing old equipment.

The county supervisors also cut back on pay increases for the county's roughly 1,000 employes, who now earn an average of $12,000 a year.

The employes will get a 4.8 percent "merit" pay increase, which has become automatic, but a proposed 4.8 percent cost-of-living increase was killed. Instead, the board approved a one-time bonus of $200 - about 1.6 per cent for the average worker.

Two departments came out particularly well in the budget process. The police budget was increased 20 per cent to $4.4 million with 18 new positions added to the current 166. The [WORD ILLEGIBLE] economic development office was given a 50 percent increase to $86,000 a year. Another major winner was the county contingency fund, which was increased by $250,000 to $652,000.

One budget item allocates a contribution of $6,100 to the Health Systems Agency of Northern Virginia, the governmental coordinating body for health resources in the area, but the board specified that it is not to be paid if the agency persists in its plans to lobby for use of state and federal funds for abortions.

The county was helped in holding the tax line by an increase through growth of about $1.5 million in real estate tax receipts and an additional $1.8 million from assorted fees, licenses, permits and services, according to finance director Garnett C. Ball Jr.

The county also was aided by a peculiar circumstance arising from a previous board's use of separate tax and fiscal years to keep taxes artifically low.

That had resulted in heavy shorterm borrowing at the end of each fiscal year - June 30. Moving to eliminate that expensive bulge, the board last year raised real estate taxes by 90 percents to the $5.25 level, which was continued yesterday. In essence, the 20 percent increase in 1977 made it possible to avoid an increase this year.

The board, which has split sharply on almost every issue, remained far from unanimous yesterday. On a vote to approve a $25.9 million budget for the county government (excluding schools), Supervisors Kathleen Seefeldt and James Byrd voted no. Byrd was opposed to an increase in county employes while Seefeldt wanted money taken out of the county budget and put in the school budget.

The school proposal was passed by only 4 to 3 as Byrd opposed the $76.1 million budget as too much and Seefeldt and James McCoart attacked it as too little.

The board also approved a variety of other taxes and fire, mosquito, recreation and street light levies, keeping the personal property tax at $4 per $100 of value and setting a 1-cent tax on "all horses, mules and other kindred animals.