Directors of Suburban Bancorporation yesterday raised the bank holding company's regular quarterly dividend from 30 to 35 cents a share. The increased dividend is payable March 30 to stockholders of record March 23.

Chairman and Chief Executive Officer Robert F. Tardio stated that the board's action "reflects our confidence in Bancorporation's future despite the prospect of some difficulties in the general economy during the coming months.

"However, stockholders ought not to count on a yearend extra cash dividend," Tardio warned. "That will depend entirely in the prevailing situation next December." The corporation had declared extra dividends of 20 cents a share in 1978 and 10 cents a share in 1977.

In another development late yesterday, Joseph Richards Jr., vice chairman of the board and executive committee of Suburban Trust Co., Suburban Bancorporation's flagship bank institution, announced his resignation effective April 9.

Richard's retirement marks the end of a 30-year career in the Washington banking business. He began with Suburban National Bank as a director in 1948. After the formation of Suburban Trust, Richards subsequently served on the executive committee, as vice chairman of the board in 1960 and as chairman of the board in 1965. He served as chairman of the board of Suburban Bancorporation from 1975 to 1978.