The Fairfax County Planning Commission has endorsed plans to convert a sprawling chunk of scarred, vacant land near Franconia into what county officials say is the largest development proposed in the county in a decade. The project, which will be worth $600 million, is expected to receive final approval from the county Board of Supervisors Monday.

On 1,170 acres containing dirt-bike trails and worked-out gravel pits known as the Lehigh tract, developers propose to build a community roughly the size of the Town of Vienna or Montgomery Village.

County officials say they hope that the project will provide a revitalized downtown to areas that have some of the oldest housing in the county.

"It's the equivalent of building a small town, not much different than Vienna or Herndon," said Sidney R. Steele, director of the Office of Comprehensive Planning in Fairfax.

The development, on two sites known as Kingstowne and Landsdowne, would include homes for some 15,000 people and an estimated 10,000 jobs.

Kingstowne, the chief portion of the project, stretches from near Franconia Road in the northeast to Beulah Street in the southwest. The smaller tract, Landsdowne, sits just off the southern foot of the main site on Telegraph Road.

If approval is granted and all goes as planned, construction could begin late this summer and some homes might be ready for occupancy by late 1986. The overall project, which in addition to a variety of housing includes extensive office and industrial sites, a town center, a 74-acre park and a 14-acre lake, will take 10 to 15 years to complete, depending on a variety of factors, principally the economy, according to planners.

"I'm not sure I'd say the area is the backside of the county, but it sure isn't the front side," said Alvin D. Hall, vice president of Miller & Smith Inc., the developer of the project. "This is going to be a major change for Lee District, a downtown area they can be proud of. It will be a completely different community," he said.

The Lehigh site, much of it wooded, is the centerpiece of just two or three major undeveloped areas of Fairfax County that are targeted for heavy building. It has stayed undeveloped until now, officials and developers said, because of the tremendous cost of constructing such a project, as well as a number of environmental hurdles -- including the difficult-to-build-on marine clay -- that must be addressed before building can begin.

Just before midnight Tuesday, the planning commission recommended that an extensive package of rezonings for the project be approved, and sent it on to the county Board of Supervisors, which has final say in the case.

No citizens appeared before the planning commission to speak against the plan, which is unusual for a proposal of this magnitude. Fairfax officials say that extensive consultation among the developers, county planners and citizens' groups during the past year has worked the principal chinks out of the proposal.

Miller & Smith Inc., the McLean-based developer for the site, paid $15 million over the past several years for the land involved. The company has promised to make more than $35 million in infrastructure improvements on and around the site.

Those improvements include about $10 million in road construction that county officials say is crucial to the future of southeastern Fairfax, where traffic is now rated the No. 1 problem. Hall said the firm plans to build more than eight miles of public roads.

But county officials acknowledge that the developer's $10 million contribution is a small fraction of the amount of funding needed from other sources -- either the state or a bond floated by the county -- to ensure that already nightmarish traffic in the area does not get worse.

"The key is going to be timing," said Robert L. Moore, a transportation planner who has worked on the proposal extensively. "If this development proceeds extremely rapidly it's possible there could be a lag on the highway side."

The final plan to be presented to the county board Monday is the product of months of negotiating between the developer and county planners. Miller & Smith made 155 proffers -- commitments to provide certain amenities -- in the Kingstowne and Landsdowne development -- a record, according to county planners. Those commitments include extensive environmental safeguards, road changes and recreational facilities.

Miller & Smith also pledged to finance the design for an interchange at the intersection of South Van Dorn Street and Franconia Road, a junction already strained by traffic that would be further pressed by the Kingstowne and Landsdowne projects.

The design work, which would cost about $500,000, would be a first step in building an interchange that could cost about $10 million. County officials say finding the money to build that intersection is important to the development of the area.