Fairfax County has won the right to grant tax breaks to owners of small farms in an effort to protect some open space from rapidly encroaching development.

The Virginia General Assembly approved a bill last week that will allow county supervisors to tax farms of 25 acres or more based on how much they produce, rather than what their land could fetch if sold for development. Current state law allows tax breaks only for farm areas of 500 acres or more, a provision that no longer has much value in suburban Fairfax.

The new law, if signed by Gov. Charles S. Robb, also will allow Fairfax to levy stiffer fines against farmers who develop their land after having enjoyed tax breaks. And for the first time the county will be able to withhold tax breaks from land that is zoned for future commercial growth.

"I'm very, very pleased," said Supervisor Marie Travesky, whose sprawling Springfield district contains most of the county's farmland. "I think a lot of people will show how strongly they feel about keeping farms in Fairfax now that they have that opportunity."

Once a major dairy and vegetable producer, Fairfax County has lost farmland steadily since World War II until now less than 8 percent of its acreage is farmed, according to a county study completed last spring. Development pressures, including rising taxes, have rendered agriculture less and less profitable. Most of the county's remaining farmers no longer consider agriculture to be their principal occupation, the study showed.

The county has approved two agricultural districts of more than 500 acres, one near Great Falls and one on Mason Neck, south of Mount Vernon. Only two or three other areas in Fairfax have enough contiguous farmland to qualify for tax breaks under current state law, according to Phoebe Kilby, the county's farm preservation expert.

The new law would allow the board to grant tax breaks to anyone with 25 acres or more of forest or farmland. "There are a lot of landholders in the western end that would like to hold onto their land, and they do not have the large acreage," Travesky said. "They can't even hold onto 25 acres without some tax breaks."

The county study identified 422 remaining farms. A number of those are smaller than 25 acres.

To ensure that landowners do not abuse the tax breaks while waiting for a profitable moment to develop, state law requires farmers to pay five back years of taxes if they develop land that has been held in agricultural districts. The new law adds an additional penalty of two years of taxes at full market value, which Kilby said would further discourage landowners who do not genuinely intend to preserve their farms.

The Senate passed the bill 20 to 18 on Thursday, with some senators objecting that the county was trying to slow legitimate development. The House of Delegates approved the measure by a 67-to-21 margin.