The Loudoun County Board of Supervisors this week deferred action until Jan. 20 on what it calls "the most important single step toward economic development this board will make all year," an amendment to the zoning code that could nearly triple the size of a commercial building that a developer can put up on his land.

According to Supervisor Frank Raflo, who with Supervisor Andrew Bird served on the ad hoc committee that wrote the amendment, the new regulations would allow developers to increase the use of land by "sitting down and reasoning with county officials" about public improvements on roads, water and sewers that the developer would make in exchange for the higher allowed usage.

Some officials said the changes are being contemplated to accommodate future commercial development around the proposed Center for Innovative Technology that will straddle the Fairfax-Loudoun border near the junction of the Dulles Toll Road and Rte. 28. A consortium of 13 major businesses plan to construct a research and development center on the 35-acre tract that is near Dulles International Airport.

The board deferred action, it said, to give its members time to study the 10-page amendment and to give the public time to respond to it. "We've been caught before," said newly elected Board Chairman James Brownell. "We think everyone understands something, we act on it and then we get complaints. We hope this will prevent that."

The amendment refers specifically to the floor area ratio, which is to commercial and industrial development what housing unit density is to residential development. Current zoning laws allow a floor area ratio of 0.4. That is, if a developer has 10,000 square feet of land, he can have 4,000 square feet of floor area in his building. The amendment would allow a ratio of 0.6 with a range of up to 1.0 if plans are approved. Development plans would be considered in units of 20 acres, officials said.

In addition, the board approved four out of five recommendations from its Education Committee that could put a $50 million bond referendum issue on the ballot sometime this year. The bond issue would include funds for the construction of five schools and air conditioning for three high schools: Broad Run, Loudoun County and Loudoun Valley.

The board deferred action on a fifth recommendation that would have authorized school officials to hire an architect to design the high school, middle school and three elementary schools proposed. The supervisors said they want to determine the public's response to the bond issue before hiring an architect for an estimated $1.6 million. County Administrator Philip A. Bolen noted later that because of Loudoun's increasing population, the schools will have to be built anyway. "Even if the bond issue fails, the need will still be there. The money wouldn't be wasted."

According to school administration figures, the five new schools should be built by 1990. Enrollment projections on which the building program is based were derived by building permits approved by the county and from residential construction anticipated over the next 10 years. According to county figures, projected growth in eastern Loudoun is 1,059 housing units per year. Projected elementary school enrollments for the area have nearly doubled since 1984, from 744 to 1,414. Total capacity for the five new schools would be nearly 5,000 students, school officials said.

Schools have been successfully funded in Loudoun County through bond referendums with the county's AA rating from two prestigious bond rating houses in New York.