The National Restaurant Association, one of the nation's 20 largest trade groups with an annual budget of $6.5 million, has decided to move to the Washington area from Chicago.

According to association spokesman Michael Horowitz, the group's board voted for the move at a recent meeting. The only remaining approval needed is by the association membership at a convention in May. Horowitz said "there is little chance it won't go through."

Founded in 1919, the Restaurant Association is the industry's leading trade group with a membership representing more than 100,000 food service establishments.

Horowitz said yesterday his group is looking at possible sites for its offices in D.C. and in the Maryland and Virginia suburbs.

Currently, the restaurant group has a 25-person Washington staff and 75 persons at the Chicago headquarters. After the move takes place -- about mid-1980 -- the numbers will be reversed with about 75 persons here and 25 remaining in Chicago who will concentrate on educational and convention operations.

Each May, the association's convention and exposition attracts about 100,000 persons to Chicago's McCormick Place. Because few cities have facilities for such a large crowd, the show will continue in Chicago along with the trade group's convention people. Other employes will be asked to move here, Horowitz said.

Separately, Fairfax County economic development officials have made public a study about the impact of the trade association business in their county.

There are 125 trade or professional groups now based in the county compared with 2 a decade ago, the Economic Development Authority said. These organizations employ 3,000 persons and have annual budgets of $40 million. Throughout the area, trade association employment is estimated at more than 40,000. making this business one of the largest in Washington's private sector.