Montgomery County, already planning to build 13 schools in the booming upper county by 1993, will need to add six more to accommodate the legacy of recent economic growth -- children of young families, school officials announced yesterday.

School Superintendent Harry Pitt is submitting a budget request that includes $47 million for this additional construction over the next six years, as Montgomery moves into a decade in which 11,000 births are expected a year. Montgomery has experienced a steady growth of births since a modern-day low of 6,478 was set in 1975.

There are 96,454 students currently enrolled in 154 schools in the county; by the fall of 1993, nearly 120,500 are expected to show up for classes.

Pitt's request for six additional schools, scheduled to be reviewed in a series of school board hearings this month, would boost to $420 million the total expenditures for school construction and rehabilitation over the next six years. The proposed addition of 19 new schools comes after the county closed more than 60 schools in the past decade.

Pitt and County Council member Michael L. Subin, chairman of the Education Committee, said yesterday that Montgomery might expect to get up to $12 million annually in state aid for school construction. The state once pledged to pay all those costs, but is now shifting more of the burden to the counties. Montgomery currently gets about $9 million annually from the state for school construction.

The half-dozen new facilities Pitt is proposing for Montgomery, including five elementary schools and one middle school, would be scattered across the midsection of the county, from Germantown in the high-growth I-270 corridor, to the eastern tip of the county, where considerable housing construction is under way along Rte. 29.

Pitt said he wants to add elementary schools to the Germantown, North Springbrook, Redland, Quince Orchard and Olney areas. Two new elementaries opened in Germantown this year. The new middle school would be in the Briggs Chaney area.

County officials have anticipated for several years that demographic changes would require still more school construction, Subin said. "We're having 3,000 additional, unexpected live births a year just from the population that's here," he said, "If the economy stays good, people will continue having kids. We know that the zero-to-5 age bracket is growing faster than earlier expected."

Subin said that while he was not ruling out the possibility that new schools would have to be built, he believed the school system would be urged to see what other measures it could take. Among those would be redrawing school boundaries and adding portable classrooms, he said.

From 1972 to 1981, the county school population declined, and more than 60 schools were closed. But starting in 1982, after the economy picked up and the county government eased its moratorium on new housing construction in parts of the county, school enrollment began to climb.

Since then, county officials have been under particular pressure to add schools in the upper county, where new schools typically are crowded as soon as they open. Planning to build now "will enable us to keep pace with the number of kids coming into the schools," school system spokesman William Henry said.

Because the closed schools are concentrated in the lower county, where populations tend to be more stable, the school system has plans to reopen only one, Cloverly Elementary in the Fairland area of the eastern county. The closed facilities are being rented for a variety of uses, including private schools.

The schools were closed at a time when there was "a baby bust," school system demographic planner Bruce Crispell said yesterday. "The economy wasn't strong, and families were not having babies."

Since then, he said, the county's economy has been growing rapidly, prompting "a real surge in having families. People are also moving here in part because of the school system." The majority of those new residents are of child-producing years, he said.

At the same time, the county has "set an all-time record for housing completions, fueled by job growth," with 10,739 units built last year, Crispell said. Most of Montgomery's undeveloped or newly developed land is in the northern and eastern county, where the new schools are to be built.

The bottom line as far as school officials are concerned is that there will be "a wave of younger children moving into the system in the late 1980s and in the 1990s," he said. After 1990, nearly every grade in the school system will increase in enrollment.