Riggs National Bank -- the largest commerical banking business in metropolitan Washington, with assets of close to $3 billion -- announced plans last night to reorganize its corporate structure into a bank holding company.

The announcement came less than a week after the Carter administration endorsed proposals that would allow bank holding companies to expand on a regional basis -- a prospect currently denied Riggs and other D.C.-based banks by regulations prohibiting new branches across state lines.

In an interview, Riggs President Daniel Callahan III said bank management and directors have studied and discussed the bank holding company idea for many years and that yesterday's action was not tied directly to the adiministration's policy decision.

"More and more banks are doing more and more things." said Callahan of the swiftly changing competitive enviroment for the nation's financial institutions. "A holding company will provide flexibility in financing opportunities ot have other authorized corporations under the wing of the firm."

Whatever changes may take place in banking laws and regulations, said Riggs Chairman Vincent Burke, Jr., the holding company structure will offer more opportunities for expanding business into activities directly or indirectly related to banking through formation of new subsidiaries or the acquistion of established companies "in the event that attractive opportunities should develop."

Burke said Riggs has no acquistions orf expansion plans on the drawing boards, although the bank already has a major subsidiary, Central Charge Service, the largest regional credit card business.

The principal competitors of Riggs that are based in the Washington region -- American Security Bank inD.D., Suburan Trust Co. of Hyattsville, First Virginia of Falls Church, and the three First American Banks in D.C., Maryland and Virginia -- already are owned by holding companies that have diverse and growing subsidiaries.

In addition, the major bank firms in Maryland and Virginia all are holding companies -- based in Baltimore, Richmond, Norfolk and Roanoke -- and all of them are expanding in this market, sometimes across state lines. Although direct retail bank branching is prohibited outside a home state, bank company subsidiaries engaged in such related businesses as lending, mortgage banking and leasing can be located anywhere.

For example, two huge California banks, Security Pacific and BankAmerica, have active personal finance subsidiaries in the Washington market.

Burke said the proposed reorganization would involve an exchange of bank stock for stock of the holding company and that Riggs (as a subsidiary of the parent firm) would continue to operate as a national bank.

Riggs directors still must approve a definitive plan of reorganization, and the change must be approved by owners of two-thirds of Riggs stock. Regulatory agency approval and favorable tax rulings also are required.

In another development, Riggs directors yesterday approved a regular quarterly dividend of 70 cents a share for common stock, payable Oct. 15 to owners of records Sept. 30.