For the first time in four years, the number of serious crimes in Fairfax County increased slightly in 1984, according to figures released yesterday by the county police.

Police attribute the 1.26 percent rise -- up from 20,651 in 1983 to 20,912 in 1984 -- to the increase in "potential crime targets," such as the "increase in population, housing units and commercial development," said Warren Carmichael, Fairfax County public information officer.

Carmichael said that the county's crime rate is actually lower for 1984 because the county's growth in population offset the increase in the number of offenses.

The crime rate is the frequency of serious crimes per 100,000 persons in the county. The seven crimes computed are: murder, rape, robbery, aggravated assault, burglary, larceny and motor vehicle theft.

Nationwide, crime figures have declined since 1982 and preliminary figures for the first six months of 1984 show a 7 percent drop, according to FBI statistics.

The number of crimes fell in most Washington area jurisdictions over the last year. The number of crimes did rise slightly in Montgomery County, up 4.1 percent in 1984 compared to 1983.

In Arlington County, serious crimes dropped .2 percent in 1984. For the same period, crime was down 5.8 percent in Alexandria. In the District, through October 1984, the number of crimes was down 6 percent, and in Prince George's County crime was down 5.6 percent through November 1984.

Fairfax has had to contend with booming commercial development and a flourishing population.

"What this means is that during the last four years we have added the equivalent of a medium-sized city larger than either Charlottesville or Petersburg, while simultaneously experiencing a decrease in crime greater than national averages," said Col. Carroll D. Buracker, Fairfax County Chief of Police.

"Crime rate, as opposed to actual numbers of offenses, is the only true comparative measurement of crime, and as long as it continues to decrease, we are making progress," Buracker said. "Given the population increase of the county, I'm elated with the results," he said.

An increase in larcenies, from 15,079 to 15,499, accounted for much of the increased number of offenses, the Fairfax report said.

Robberies increased from 364 in 1983 to 442 in 1984, the largest percentage increase of any crime.

"We had more groups, three or four individuals who were responsible for half a dozen or more robberies," Buracker noted. Rapes decreased 10 percent from 68 in 1983 to 61 in 1984.

Buracker attributed some of the decrease in certain crimes to "community participation" in such programs as Neighborhood Watch. Burglaries declined for the fourth consecutive year, dropping 8.44 percent.