Back in the early 1960s, when all of Loudoun County was farm country, the Federal Aviation Administration decided to run a sewer line from Washington's Blue Plains treatment plant to Dulles Airport.

Today, Loudoun is still trying to cope with the effects of that decision, which brought sewer service not only to the airport but to eastern Loudoun County as well. With the sewer came a building boom that has pushed out the cows and jammed more than 7,500 homes into a space that, in 1960, held only 500. By 1987, county planners say, that number should double.

Worried about the impact of a large influx of families on Loudoun's roads and schools, county officials are trying to control growth by allowing small planned communities, along the lines of Fairfax's Reston, to be built east of Rte. 28 and north of Dulles Airport.

Those planned communities are now fueling the county's rapid growth rate. Yet their supporters say they are also helping Loudoun cope with an unprecedented wave of new suburbanites that otherwise might overwhelm the county. Under a complicated bartering system developed by Loudoun officials, developers are agreeing to provide amenities like road improvements and school sites in exchange for increased housing densities.

"We call them 'mini-Restons' but they are not like Reston that much," said Loudoun County's chief planner, William Keefe. "They don't have employment opportunities. They are essentially bedroom communities with convenience shopping centers, pools and pathways. But they allow us to get some of the things the county needs."

With a low tax rate and cheap land, Loudoun County became attractive to builders hoping to lure people who work in neighboring Fairfax County. Now that the housing market is improving, developers say they are beginning work on three of the biggest planned communities in eastern Loudoun. When completed around the end of the decade, the three will cover the rolling hillsides between Rte. 7 and the Potomac River with thousands of new homes.

Construction on CountrySide, a development of the Hartford Insurance Company, started recently after 10 years of court battles and dickering with the county. When complete, it will consist of 2,500 homes with two elementary schools, a middle school, a fire station, a library, and a recreation center.

The developer is providing sites for schools and county services, but taxpayers will have to pay for their construction.

"Because CountrySide was settled by the courts, we didn't have that much say in what kinds of improvements they would provide," said Jeffrey A. Nein, Loudoun County's chief for land development planning. "We hope to have more control over the developments that have come in since we adopted the management plan that outlines the county's needs."

At Cascades, a 1,000-home development planned for the eastern-most corner of the county just west of Great Falls, county planning officials granted developer Juan Montourian an increase in density of 200 homes in return for a promise to help pay for the completion of the Rte. 28 loop, a four-lane highway that will extend Rte. 28 north of Rte. 7 to serve the planned communities. Montouri has also agreed to provide a school site.

And at nearby Cameron Glen, a massive subdivision planned to include 3,000 homes, the Sunrise Development Corp. has offered to provide school sites, community amenities and financial assistance for road construction. But county officials say they are still wary of the corporation's proposal to build a regional shopping mall on Rte. 7 because they fear it could cause major tie-ups on the county's main commuter link with Washington.

"I think that for most of the public services, law enforcement, fire and rescue, the county is in pretty good shape," said Supervisor Travis L. Sample (D-Dulles). "The real crisis is transportation, the logistics of moving all these new people."

County officials say there will be a critical need for new roads as the housing boom continues. But while developers say they want to make sure their new communities will be served by adequate highways, they also say they will not abandon their plans if the roads are not built.

The extension of the Rte. 28 loop into Fairfax County has been opposed by the Fairfax County Board because it would run through an established community in Great Falls. Still, Montouri said through a spokesman this week that he will build with or without the Rte. 28 loop extension.

"We've offered to pay for the road, and it will be fairly shortsighted of them if they don't let us," said Richard Hobson, a Fairfax attorney who represents Montouri.