Fairfax County Board Vice Chairman Martha V. Pennino says she has a novel idea to help local planners look at long-range land-use issues.

Pennino maintains that the county should establish a maximum population of just over 1 million, based on what she considers to be a reasonable average of four persons per acre of developable land in the county. Fairfax County's population, already about 680,000, has grown at a dizzying pace so far this decade.

Pennino, known locally as "Mother Fairfax" and the supervisor who oversaw the development of Reston, the planned community in western Fairfax, said the Board of Supervisors cannot adequately plan the number of offices, roads, schools, recreation areas and such that it will need without having an idea of how many people will be living in the county in, say, the year 2020.

"It's almost ridiculous to plan for the size of your house if you don't know what the size of your family's going to be," she said.

But when Pennino broached the idea at yesterday's county board meeting -- and asked for a staff report on the question in the near future -- confusion erupted.

"I'm puzzled," said Supervisor Thomas M. Davis III (R-Mason).

"We started off talking about office buildings. And now are we talking about people per square inch? The year 2020? I'm not sure our staff can come up with any plan for the year 2020 in 60 to 90 days."

The bewilderment continued until a top county bureaucrat arose to inform the board that Pennino's concept is known, in the academic parlance of planning, as "a capacity analysis approach." The bureaucrat said he'd be glad to report back to the board on its planning options this fall.