The Leesburg Town Council, reacting to the onslaught of development activity the town has experienced in the past year, added a fourth planner to its planning department when it approved its $4.4 million 1985-86 city budget last week.

According to chief planner Martha Semmes, development activity appears to have increased significantly within the past two weeks alone. The staff of three planners and administrative assistants responded to more than a dozen development inquiries, seven of them in one day. "It's a building explosion," she said. "We really needed that fourth planner."

According to a staff report, Leesburg's average annual growth rate is 5.4 percent. Estimated figures show a population of 10,900 by the end of this year, nearly 14,000 by 1990 and 16,200 in 1995. In housing alone, the staff projects construction of 4,200 units estimated for 1985 to 6,350 units in 10 years. The labor force is expected to nearly double by 1995, to 9,000 from more than 5,000 at the end of 1984.

Development activity this month included a request for six subdivision plats for nearly 200 housing unit lots; an office park, and more than 22,000 square feet for retail shops. Development plans reviewed by the staff included 820 apartment units east on Rte. 7, nearly 22,000 square feet for a self-storage warehouse and another 40,000 square feet for offices, retail shops and convenience stores.

"The economy is generally good, and Leesburg is benefiting from that," Semmes said. "And land is still cheaper here -- from $15,000 to $20,000 an acre for residential purposes -- than it is in Fairfax."

Other inquiries in recent weeks have included Realtors with potential clients looking for investment opportunities and a public service company attempting to catch up with new demands for utilities. In addition, Semmes said, there was a rezoning request from a Swiss company that wants to build a planned community on farm land it owns and a variance request from a developer planning to build 820 apartments, who is seeking relief from a requirement that the units be set back 150 feet from a nearby single-family residential area.

There also were five Board of Architectural Review requests, in which developers hope to improve existing buildings or build new ones compatible with the 235-year-old town's historic district.

"We hoped to attract light industry and high-tech business to the east end of Leesburg on the Rte. 7 strip to Goose Creek," said Assistant Town Manager Stephen Owen.

That strip, Owen said, will be zoned for an industrial park. Another industrial park may be going up soon next to Leesburg's fast-growing airport. The Richlynn Development Corp., which owns a nearly 78-acre strip there, has asked the town to rezone the parcel from agricultural-residential to planned development industrial park. The airport itself also is changing. Two old buildings are being torn down to make way for a second apron to accommodate nearly 60 parked planes, and a thousand-foot extension of the runway, which the town hopes will lure corporate jets, is on the drawing board.

A nearly $8 million expansion of the town's water treatment plant, now under construction, was planned with an eye toward growth. In addition, Owen said, another police officer was added to the rolls this year for the same reason, bringing the total to 19 officers.

One of the biggest problems facing any growing jurisdiction is roads, and Leesburg is no different. As the result of a change in the funding formula approved this year by the General Assembly, the town will receive nearly $226,000 for road maintenance and $361,000 for new construction, up $130,000 from last year.

Rte. 7, the town's main link to the Washington metropolitan area, will be the focus of much of the new construction; a connector road to Rte. 7 will be built and an interchange will be constructed at Rte. 7 and Cardinal Park Drive. In addition, Owen said, a minor arterial road to the airport is in the long-term road plan.

Although providing schools for the burgeoning population is a county responsibility, Semmes' department works closely with developers to acquire land that can be used for new school buildings. The Swiss firm, Beus Inc., has provided the town with a 15-acre site for an elementary school; the Exeter Housing Development, which has applied for nearly 180 subdivision plats, offered 22 acres for a middle school, but the School Board said it needed 30 and turned down the offer. Semmes predicted that within 10 years, five elementary schools, two high schools and two middle schools will be built to accommodate Leesburg's expected population increase.

The booming growth of Leesburg also brings a need for expansion of other services. According to the former fire chief of the 34-member volunteer fire department, Terry Frye, "We're going to have to go to a paid department in five years if this town keeps growing." Frye said he hopes the Town Council will insist that all new commercial buildings be equipped with "the best fire protection equipment available."

One Leesburg resident, dismayed at the growth around him, was determined that a big chunk of farm land inside the town limits not be developed. William Rust donated 140 acres to Leesburg with the stipulation that it not be used for anything other than a park. "We're delighted," Semmes said. "In 20 years, we're going to be really glad we have this much open space."