The AFL-CIO proposed today to tax its 13.5 million members an additional $4.8 million a year to make up for membership losses and pay off construction bills for its Washington-area buildings.

The proposal was approved unanimously by the federation's executive council and is expected to be ratified by delegates to the AFL-CIO's biennial convention, which opens here today.

The increase would amount to only three cents a month per member, but would add up to a 60-per-cent increase in members' support of the federation's activities over the past two years. In 1975, the monthly fee was raised from 10 to 13 cents. The new fee, starting next January, would be 16 cents.

The per-capita fee, which is an addition to dues owed by members to their individual unions, has to be raised in part because the federation has been losing members - about 500,000 over the past year - according to AFL-CIO spokesman Albert J. Zack. The losses come largely because of job cutbacks caused by economic conditions, he said.

The federation faces a six-month $475,000 deficit for the end of 1977, Zack said.

About one-third of the money will be used to pay off a $3.5-million line of credit used over the past five years to finance an addition to the AFL-CIO headquarters at 815 16 St. NW, and the acquisition of the George Meany Center for Labor Studies in Silver Spring, Zack said.

Although the convention's main concern is foreign trade and its effect on American jobs, there also is an undercurrent of concern about organizing efforts within the labor movement, from construction workers to professionals.

With considerable fanfare and a personal blessing from AFL-CIO president George Meany, 26 unions representing 1.5 million professional and other white-collar workers today announced formation of a new AFL-CIO Department for Professional Employees - a counterpart to existing departments for industrial workers, construction workers and other component parts of the traditionally blue-collar labor federation.

Albert Shanker, president of the American Federation of Teachers and head of the new department, said it will attempt to "reach the huge number of workers who are really workers but are afraid to admit they are workers."

A fact sheet prepared by the new department asserts that "union membership is appropriate for practically every kind of professional, employed in every segment of the economy," and lists jobs ranging from physicians and scientists to musicians and writers as prospective organizing targets."