More than 100,000 federal and military retirees in the Washington area will get a 3.1 percent cost-of-living adjustment (COLA) in January. It will be the smallest retiree COLA in more than 20 years.

The increase, officially set yesterday based on the consumer price index, also goes to persons receiving Social Security. The raise will show up in the January checks of civil service, military and Social Security retirees. Federal workers will not get the increase.

The modest increase reflects the decline in the rate of inflation and changes in government pension policies. At one time retirees got a cost-of-living adjustment plus 1 percent, twice a year. In 1974 they got three raises of 5.5 percent, 6.4 percent and 5.5 percent.

Last January retirees got a 3.5 percent raise. Congress and the White House blocked any further increase in 1984. The previous year retirees got 3.9 percent and in 1982 -- a period of high inflation -- they got an 8.7 percent increase.

The full COLA raises, which the Reagan administration has sought to delay or cut back, are guaranteed by law. COLAs are based on the rise in living costs as measured by the consumer price index from the third quarter (July, August and September) of 1985 over the third quarter of last year.

Congress is considering two proposals that would limit COLAs for persons retiring in the future to less than the full increase in inflation. But neither of them would affect the full COLAs guaranteed persons who are already retired.