Fairfax County's 10 members of the House of Delegates have agreed on a measure that ought to be called the "Incumbents' Preservation Act."

It's the answer to the problem of how Fairfax County, Fairfax City and Falls Church can share the 12 house members the 1980 Census says they should have and avoid retiring any incumbent in the process.

In most places around the state, the House is divided, as are most city councils, into single-member districts -- a concept that members of the Fairfax delegation pay homage to every election year.

It's a wonderful concept, they say, one that would cut the cost of running for office and give better service to voters in a part of Virginia that often feels isolated from state government.

But there's one political landmine buried under their rhetoric, and it didn't take a long look at census maps for the 10 Fairfax incumbents to discover it: They're neighbors.

And that would mean somebody would have to quit, move to another district -- or face a neighbor-legislator at the polls. So despite their frequent complaints about the long hours and the low pay of their part-time jobs, the legislators, well in advance of a special redistricting session, have reached an agreement to preserve themselves.

"The way we've agreed to work this out, everyone fits in," said Del. Vincent Callahan (R-Fairfax), the only House member from Fairfax on the Privileges and Elections Committee whose members will decide how to cut up old territory into new districts.

Because the population in Northern Virginia has increased since the last reapportionment 10 years ago, the region is expected to pick up at least three new seats in the House.

Two of those new seats will go to Fairfax County. The tentative decision reached by the Privileges and Elections Committee will change Fairfax County's representation from two 5-member districts to four 3-member districts.

The four new districts would be created by dividing the county into four districts of approximately equal populations.

The current 18th District would be redrawn by separating northern Fairfax County along a line following Rte. 7 from the Loudoun County border to the Capital Beltway. The line then would follow the Beltway south to Rte. 236, which is the current dividing line between the 15th and 19th districts.

That would put Falls Church, Mason, half of Providence and all of Dranesville (except Herndon) in one district and leave Fairfax City, Centreville, Herndon and the other half of Providence in another.

More importantly to the legislators, it would put Callahan and fellow Republican Del. Martin H. Perper, who both live in McLean, in one district with three seats and delegates John S. Buckley, John H. Rust Jr., both Republicans, and Del. Dorothy S. McDiarmid, a Democrat, in another three-member district.

More troublesome was drawing the line in the southern half of the county where two legislators -- delegates Robert E. Harris and James E. Dillard II, both Republicans -- live within a few blocks of each other.

The result: The 19th District will be cut in two along a line just west of Shirley Highway. That would place Del. Gladys B. Keating, the only Democrat in the district, into a new three-member district with Del. Warren E. Barry, who lives just west of the highway. Delegates Harris, Dillard and Lawrence D. Pratt would be in the three-member southwestern district.

That district would gain two precincts south of Fairfax City -- Villa and Woodson -- which now are in the northern part of the 18th district. There might be other, minor changes, but basically, the Lee and Mount Vernon magisterial districts of Fairfax would be in one area while Springfield and Annandale would be in the other, Callahan said.

Callahan said the key to success of the plan is Falls Church, which he wants to keep in a district with eastern Fairfax. If Falls Church is merged with Arlington in another district, it would create an Arlington-falls Church district with the exact population needed to keep the three House seats Arlington now has. But that would foul the Fairfax plan.

"That could set the whole thing off," said Callahan.But he added, "Arlington doesn't want Falls Church and Falls Church doesn't want Arlington."

Delegate Perper, for one, believes single-member districts are the best way to give voters equal representation. "I think everybody would like single-member districts," Perper said. "It's much better for people to have single-member districts. Your ability to provide services is greatly heightened."

But Perper conceded the proximity of House members' homes made the concept politically difficult.

"It's a very practical problem. Most especially do you have that problem in the southern part of the 19th District where Dillard, Harris and Pratt all live in a small area."