The Loudoun County Board of Supervisors approved a major change in the county's zoning rules yesterday that will nearly triple the maximum allowable size of commercial projects, letting Loudoun, supporters say, benefit from the boom in high-technology developments now taking place around Dulles International Airport.

The change will increase by 50 percent the amount of floor space developers are permitted to build on a given amount of land. It also will allow the possibility of further increasing this ratio of floor space to land if a developer is willing to supply the county with necessary public improvements, such as roads and sewers.

The change was drafted in response to the avalanche of new development expected along the Loudoun-Fairfax County border because of the proposed Center for Innovative Technology, a state-sponsored complex to foster high-technology research.

In approving the measure by a vote of 7 to 1, the board compromised on development, which has often put its members at sharp odds. Supervisors Frank Raflo and Andrew R. Bird III, two authors of the proposal, have clashed over the problem of attracting growth versus saving Loudoun's rural character, with Bird favoring more rapid expansion.

Raflo said the new measure will allow Loudoun to do for commercial developments what it has long sought to do on residential projects: exact concessions from developers in exchange for zoning approvals. By forcing developers to explain why they wish to have more space and what they are willing to give the county in return, Raflo said, "it gives us the versatility we need to be positive on economic growth."

Bird trumpeted the measure as a "watershed change in Loudoun's attitude toward really getting economic growth off the ground."

Not everyone was convinced. Mark Fried, a Fairfax developer attending the board session, said that even with the changes, Loudoun's commercial development rules are more restrictive than others in the area. Without further relaxation, Fried said, Loudoun will see itself losing to Fairfax County lucrative high-technology projects along Rte. 28.

Fairfax, for example, allows developers to include land that lies in flood plains when calculating the ratio between floor space and total property. The board yesterday beat back a proposal from Steve W. Stockman that would have allowed Loudoun developers to do the same thing.

Supervisor Ann B. Kavanagh, the only board member voting against the measure, said that the criteria by which the county would allow a developer to increase his floor space were too vague. She said Loudoun's recent history shows that developers usually insist on building at maximum levels, even if they have offered little to the county in return.