A Florida developer said yesterday he plans to build a $220-million resort and conference complex along the Potamac River in Loudoun County, a project that Loudon officials said could become the county's largest tax-payer and employer.

The 1,280-acre development would include 2,300 hotel units, two 18-hole golf courses, 60 indoor tennis courts and 40 to 60 outdoor tennis courts under plans to be unveiled formally Friday in Leesburg by developer Harvey P. Jones. The development would be built over an 8-to-10-year period.

Jon M. Jones, vice president of Harvey P. Jones International Inc. said the company would apply for rezoning and development permits Friday and hoped to break ground this summer.

County officials the developer has no guarantee of approval for the project, called "Gilgarren," but they reacted highly favorably to the proposal.

Supervisor Carl F. Henrickson, whose district would contain Gilgaren, said, "My gut reaction is a favorable one. It's a good use of the property. You don't have the kind of costs you have with a housing development - primarily schools.

"Harvey Jones and his two sons and daughter-in-law were up here working on it. It's not some great big conglomerate coming in and wanting to run over Loudon County. They are willing to work with the county," Hendrickson said.

County zoning director Joseph Trocino said the cost of county services provided to the project would be low and that tax revenue from it would be high. Trocino said the proposal appeared to be an excellent use for the land, located along the Fairfax-Loudoun County line, including about 100 acres in Fairfax.

The first phase of the project, which is expected to take two years to complete, would include a 384-unit "racquet" complex, four "manor houses" with five units each and an 18-hole golf course, Jon Jones said. The racquet complex would contain 12 indoor tennis courts under translucent canopies. At this stage, the resort would employ 350 persons, he said.

Units would be sold at an average price of $70,000 to investors, mostly corporations, and would be managed by the Jones company as rentals, he said. The complex would be used primarily for corporate conferences, he said.

Five 384-unitbuildings, three 90-unit buildings and 54 five-unit buildings eventually are to be built, and 2,000 people would be employed, he said.

The Jones corporation also has developed "Innisbrook" a golfing resort near Tarpon Springs, Fla., and "Tammerron," a ski-orientated resort in Colorado.

Jon Jones said that most resort developments have "followed the sun" but that his father, Harvey, believed that a metropolitan-area resort also would be sucessful.

The Washington area was chosen because of its large number of association and corporate offices, its access to airports and the high-quality land available, Jon Jones said.

he said the corporation will pay about $4 million for the land.