The Loudoun County Board of Supervisors approved an experimental zoning ordinance yesterday devised to slow the invasion of suburban sprawl into rural areas.

The law would give landowners the option of creating hamlets, or small country villages, by grouping new houses near the center of a tract and surrounding them with open fields. The measure is a key component of the county's controversial Vision Program for western Loudoun, which attempts to guide development by providing options to traditional subdivisions.

"The suburban sprawl thing really hasn't worked very well," board Vice Chairman Charles A. Bos (D-Leesburg) said before yesterday's unanimous vote. He noted that there has been "considerable interest around the country" in the Loudoun hamlet law. County officials say the ordinance is unusual for a rural area, although the concept has been applied in suburban areas, including Fairfax County.

Much of western Loudoun is now zoned so that each house must be built on at least three acres. The hamlet option, a purely voluntary measure, would allow a landowner or developer to build five to 25 houses on lots as small as one-third of an acre, in a cluster. Eighty percent of the overall tract would have to be preserved as open space.

Some developers predict the experiment will fail because there are few financial incentives for them to use the hamlet ordinance. They say it won't let them build more houses and could cost them more for water and sewage planning than if they use standard zoning. State laws limit how localities can control development.

Ranging from Dulles International Airport and Sterling Park to the Blue Ridge Mountains and West Virginia, Loudoun County experienced the Washington area's fastest population growth in the 1980s. Although much of eastern Loudoun is built out or rezoned for development, much of the area west of Leesburg and Goose Creek is farm or timber land.

The Board of Supervisors was elected to four-year terms in 1987 with a mandate to manage growth. County officials devised Vision, which initially included proposals under which some building densities could be reduced.

In several nights of acrimonious public hearings in January 1989, farmers and developers denounced the program as an attempt to undercut their rights and their land values. Officials went back to the drawing board, scheduling an exhaustive schedule of public meetings.

Supervisors, who face election again next year, say the ordinance that was approved yesterday is the most significant of three designed to give landowners practical options to preserve open space, if not farmland. The county has a strong environmental lobby, which backed the hamlet law, and county officials tout Loudoun's pastoral qualities to prospective businesses.

Unlike some existing Loudoun hamlets, which line busy two-lane roads, new ones would have to be set back from major arteries, in part to provide an attractive setting for new house buyers and the county's current 90,000 residents.

Leesburg developer Frederick Carr said yesterday he is among area builders who are considering creating hamlets "on a limited basis." Planning consultant Marc Weiss, president of the county chapter of the Northern Virginia Building Industry Association, said the measure's continued reliance on wells and septic fields and failure to allow increased building density may limit its use drastically.

Yesterday's action "is a big step in the right direction for our Vision for western Loudoun," County Board Chairman Betty W. Tatum (D-Guilford) said after the vote.