Loudoun County supervisors, in an unusual public dispute over a major new business development, are split over the county's ability to pay for incentives to lure a $500 million United Airlines maintenance plant.

The county is one of 15 jurisdictions nationwide bidding for the United project. If the airline selects a parcel on or near Dulles International Airport property in Loudoun, the plant would become the largest single development project in county history, officials say.

Planned to open in phases beginning in 1993, the plant would employ as many as 7,000 people. Its facilities and equipment would be valued at more than half a billion dollars if fully built, according to a United spokeswoman. The airline, which has a huge reservation center near the airport, is already Loudoun's largest employer.

County officials are telling United they would improve two-lane Route 606, at a cost of from $2 million to more than $60 million, and that they would extend water and sewer lines to the maintenance facility, with a price tag of $1 million to $9 million. The government's potential revenue shortfall in the coming fiscal year is estimated at $28 million to $34 million, or about 10 percent of its needs.

"Where is it going to come from?" asked Supervisor Betsey Brown (D-Catoctin), who fears that money pegged for road projects in the rural west end of the county would be diverted to the more populated Sterling area if United chooses Loudoun.

Board Chairman Betty W. Tatum (D-Guilford) said all offers to United are contingent on approval by the full County Board, adding that the aircraft maintenance facility would be a significant economic boon to all of Loudoun.

The rift underscores some of the dilemmas facing the growing county, which stretches from the airport to the West Virginia border. Loudoun needs to increase its commercial and industrial tax base, but in the effort to attract development county officials must dig deeper into nearly empty pockets.

In addition, a county growth policy still being debated would steer most new development into the urban east and preserve open space in the west. One of the tradeoffs is that rural areas would receive fewer direct benefits such as roads and sewers.

Supervisors Brown and Thomas S. Dodson (D-Mercer), who have voted against the board majority on several key issues this year, said they don't support the offer to United because they have not been given enough information by top elected and appointed officials about the deal's potential costs and benefits.

"I wanted to see both sides of the equation," said Dodson.

If the maintenance plant is built on airport land instead of adjacent property, it could produce as little as half of the potential annual tax revenue to the county, officials say.

County Board Vice Chairman Charles A. Bos (D-Leesburg) said officials are hesitant to offer similar incentives for a football stadium because it would create fewer jobs. The Washington Redskins are reported to be considering stadium sites in the District and Loudoun.