A boundary war of sorts has erupted in Northern Virginia as the intensive residential development planned for the eastern edge of Loudoun County threatens to disrupt the tranquility of Great Falls and other areas of western Fairfax County.

The immediate dispute is over the access that Loudoun County commuters have to Va. Rte. 7, the major highway between Leesburg and Alexandria that runs east and west through Fairfax and Loudoun. But county officials say that the controversy is expected to be only the first round in a long-running feud between the two jurisdictions over development.

The Fairfax County Board of Supervisors held hearings yesterday on a proposal to restrict the access of Loudoun commuters to certain roads by blocking the roads, and to create an alternate route. The board is scheduled to vote on the proposal next Monday.

Loudoun Supervisor Carl Henrickson, a Democrat who represents the affected area, told the Fairfax board that if they insisted on such "parochialism" no regional problems could be solved. He reminded the board that Fairfax recently sued Alexandria when that city tried to restrict Baileys Crossroads residents from commuting over city roads, and he added that he hoped that Loudoun would not similarly have to resort to the courts.

The Great Falls area is composed of residential lots generally ranging from two to five acres, and Fairfax officials say the roads there were designed to support this limited development.

Loudoun, however, has planned the area bordering Fairfax for some of its most intensive residential development. Communities just west of the county line include Sterling Park, which is zoned for 4,000 units, and Sugarland Run, zoned for 1,875 units. An additional 7,800 units have been approved in the area and rezoning requests for 3,000 more units are pending.

Estimates are that the new developments would sharply increase traffic on roads in western Fairfax.

Loudoun has drawn up plans to connect this development with Rte. 7 by means of a road network in Loudoun County known as the Rte. 28 Loop. But completion of the loop will require the construction of a $3.5 million bridge over the Sugarland Run, and Loudoun lacks the necessary funds.

Without the Route 28 Loop, Fairfax officials fear that residents from the Loudoun area will clog its two-lane roads, such as Thomas Avenue, Kentland Drive and Brockman Lane.

Supervisor Nancy K. Falck, who represents the Great Falls area, has proposed a route that would block Kentland and Brockman at the county line, making them dead-end roads. Her proposal also would split Thomas Avenue in half, making the part on the western side a four-lane highway that would connect to roads in Loudoun and the part on the eastern side a short, dead-end street. The Fairfax Planning Commission has endorsed the proposal.

Falck's plan appears to have considerable support among Fairfax homeowners, but residents of Thomas Avenue strongly oppose it and many showed up at yesterday's meeting to protest. They complained that the plan would destroy their community and cut off their access to such emergency facilities as hospitals and fire stations.