Loudoun County, in a major land-use decision, yesterday won its long fight to stop shopping-center magnate Theodore N. Lerner from building what would be the first regional retail complex in the country.

The Virginia Supreme Court in Richmond overturned a lower-court decision that said the Loudoun Board of Supervisors acted illegally in 1975 in rejecting Lerner's proposed Windmill Center on Rte. 7 two miles south of Sterling Park.

"Gosh, that's great news," said board chairman Carl F. Henrickson (D-Broad Run). "Our whole land-use effort was riding on that one case."

Henrickson said he was happy for another reason. "We went up against Til Hazel, and we won," he said. John T. Hazel Jr., Lerner's attorney in the Windmill case, successfully argued two land-use cases in Fairfax County in the mid-1970s that sharply limited what Virginia localities could do to control growth.

The thrust of Virginia land-use court decision in the 1970s favored developers, especially those in fast-growing areas like Northern Virginia, where local governments tried to use their zoning powers to control growth.

It is too soon to say what impact the Windmill decision will have on those efforts, but Henrickson said flatly: "We argued that our system of representative government has been eroded in recent years by the state courts. Now representative government has been upheld."

Windmill Center, which would have been built at the southeast corner of the intersection of Rtes. 7 and 28, would have contained 1.1 million square feet of retail space.

In arguing the case, Lerner said the regional center would have generated $50 million annually in sales, with Loudoun getting $2 million annually in various taxes. The center would have provided 1,500 jobs, its backers said.

In rejecting Lerner's rezoning application for Windmill in December 1975, the supervisors simply said no, 7-0, without providing an explanation, on advice of their attorney.

Henrickson, who was not a member of the board at the time, said the center was in the wrong place at the wrong time.

"We saw what Lerner did to Tysons Corner, creating all the congestion there," he said, "and we didn't want Rte. 7 -- our primary commuter corridor --turned into another Tysons."

Lerner developed Tysons Corner Shopping Center and a number of other regional retail complexes in metropolitan Washington. His most recent project was White Flint Mall in Montgomery County near Kensington. At present, he is fighting his two Tysons partners, H. Max Ammerman and Homer Gudelsky, who want to sell the 117 acres where a Tysons II was to be built across the road from the present center.

Henrickson said public hearings held in Loudoun showed county residents were opposed to Windmill Center "6-1, even 8-1." He said the center would "have been too costly for Loudoun," straining various services in addition to roads.

The county's master plan, Henrickson says, envisions Loudoun being able to support a regional center when the county had 100,000 population, later in this decade. At present, Loudoun has about 65,000 people.

Henrickson said the plan suggested that such a center could be built north of Rte. 7 next to a yet-to-be-constructed road that is supposed to connect with the Springfield Bypass, a road still on the drawing board in Fairfax County. An alternative site would be south of Rte 7, near the Dulles Airport Access Road and Rte. 606.

The 1969 master plan did recommend a regional shopping center where Lerner wanted to build his, but Henrickson said the plan saw a 1980 population of 120,000 -- almost double what it actually is.

"The plan was just wrong," Henrickson said.