The Northern Virginia Regional Park Authority yesterday bought a 204-acre tract beside I-66 and Bull Run Regional Park in western Fairfax County as a major addition to one of Northern Virginia's most popular parks.

The apparently bargain price of $375,000 paid for the land was in part attributed to Fairfax County's controversial downzoning of land around the Occoquan Reservoir. The downzoning, which increased the size of building lots, was designed to preserve the watershed that supplies more than 600,000 area residents with drinking water; it is now the subject of a lawsuit by developers.

The land, assessed by Fairfax County at $613,000, is currently zoned for five-acre housing lots but was zoned for one-acre lots in the 1970s when developers reportedly spent close to $900,000 in development plans. A 188-unit cluster home development on the site was turned down by the County Board of Supervisors about five years ago, according to David Hobson, the park authority's chief property official who negotiated the purchase.

The $1,838-an-acre selling price is considered a bargain in an area where housing subdivision sites regularly have been selling for $6,000 to $8,000 an acre, Hobson said.

The downzoning "was the nail in the coffin," said David Feldman of Fairfax, counsel for and a principal in API Inc., the real estate investment firm that owned the land.

"But even without the downzoning the property had problems," Feldman said yesterday. "Water was two miles away," he said, and the land was unsuitable for septic tanks. "To get sewerage we had to go across parkland, which the county wouldn't agree to . . . The property needed cooperation to be developed and we didn't get it."

Park Authority Executive Director Darrell Winslow called the purchase "probably the best thing that's happened to us in a long time. It ends the possibility of houses built right next to the park and will be a crucial buffer zone" for one of the busiest public swimming pools, campgrounds and picnic areas in Northern Virginia, Winslow said.

The 5,000-acre Bull Run park stretches almost 25 miles along the historic creek where two Civil War battles were fought, from I-66 down to I-95 in Occoquan. With the 204 acres west of Centreville purchased yesterday and a 138-acre purchase three years ago along I-66, the top part of the park is now one of the largest sections and its future as a major facility is assured.

The park authority is negotiating with state highway officials for special entrance and exit ramps to the park from I-66, similar to the Dulles Access Road ramps for Wolf Trap Park for the Performing Arts. Access now is from back roads off Route 29-211.

The park authority is about to launch a new $1.7 million recreational complex at Bull Run, approved by Fairfax voters in last fall's park bond referendum, which will include a large indoor center and a soccer stadium and playing field complex.

Feldman agreed yesterday that his firm's private loss is the public's gain. The authority in effect "got a charitable contribution . . . They probably should call it the API memorial addition," he said.