The Federal Aviation Administration said yesterday that about 320,000 more Washington area residents would be exposed to significant aircraft noise by a proposed change in National Airport flight paths but that neighborhoods where noise is now the worst would get some relief.

The proposed "scatter plan," which would try to distribute aircraft noise more evenly across the area, would be safe and would save 2,400 gallons of fuel a day by shortening the round-about routes jets now fly, the FAA said in a report.

Under current rules, jets proceeding upstream reach the Cabin John area before turning. Proceeding downstream, they travel a mile or two past the Woodrow Wilson Bridge before turning. The new plan would allow jets to begin turning near Key Bridge and the Wilson Bridge.

The FAA is now accepting public comment on the plan and may implement it as an experiment for 60 to 90 days beginning this fall. Permanent adoption would require hearings and approval of the Metropolitan Council of Governments, which in 1981 requested that the plan be tested.

Currently 551,000 area residents, most of them living in the Potomac River corridor, are exposed to 75 decibels of aircraft noise for more than 30 seconds a day, the FAA estimates. That is the level at which people generally realize that noise they are hearing is coming from a jet, according to the FAA.

Under the plan, about 871,000 people would hear noise of this magnitude or greater, as planes began passing over neighborhoods that are now largely noise-free, the report said. More than half of the additional 320,000 people would be D.C. residents.

Areas of Northwest Washington north and west of the Naval Observatory would get more noise, as would Bethesda and Chevy Chase in Maryland and McLean, Arlington, Annandale and Falls Church in Virginia.

South of the airport, Marlow Heights and Oxon Hill in Maryland and Groveton and the Franconia area in Virginia would find noticeable increases in aircraft noise, the study concluded.

The Cabin John area in Maryland and neighborhoods north of McLean on the Virginia side would enjoy significant reductions in overflights and noise, according to the report, which analyzed the environmental impact of the proposed scatter plan. South of the airport, Tantallon and Camp Springs in Maryland and Fort Belvoir in Virginia would be quieter.

Under the plan, some jets would fly up the Anacostia River, as they now do. The plan would not apply to arriving jets.

The scatter plan has generated controversy since it was proposed in the mid-1970s. Supported by COG, it is opposed by the Alexandria and Arlington governments and civic groups in those jurisdictions, which contend that it would route airplanes over heavily populated areas and might compromise safety.

The plan has caused serious rifts within the Coalition on Airport Problems, an umbrella organization formed in 1979 to unite groups seeking to reduce traffic at National. CAP supports the plan on the grounds that it is unfair to concentrate noise over the river areas, but members from Arlington and Alexandria have argued against it.

Currently about 275 jet airliners as well as varying numbers of executive jets take off from National on an average weekday.