Riggs National Corp., one of the most conservative lenders in the area and the first major bank company to recognize weakness in the area's commercial real estate market, yesterday announced that it lost $56.6 million in 1990 because of bad real estate loans.

The parent of Riggs National Bank said it lost $63.7 million in 1990's fourth quarter alone, primarily because of millions of dollars that had to be set aside to protect the company against past and future real estate loan losses.

The losses prompted Riggs's board of directors to slash the quarterly dividend to shareholders in half, to 15 cents a share. The annual loss is the first Riggs has recorded since Joseph L. Allbritton became chairman in 1982.

Two area savings and loans, Beltsville-based John Hanson Savings Bank and Columbia First Bank of Virginia, also reported quarterly losses yesterday.

Allbritton, who warned in October 1989 that the Washington area was overbuilt, said yesterday that the commercial real estate market was experiencing "severe economic problems."

"We began reducing our exposure {to the market} more than two years ago, but did not predict the unprecedented decline in commercial real estate values which now pervades the area," Allbritton said in a prepared statement.

Riggs virtually stopped making real estate loans in 1988, according to banking sources, and it has been one of the most aggressive banks in dealing with troubled loans. On Friday, Riggs participated in a lawsuit to force developer Dominic F. Antonelli Jr. into involuntary bankruptcy, and it has regularly foreclosed on properties that were not generating enough profit to pay off Riggs's loans.

Despite all of that effort, regulators from the federal Office of the Comptroller of the Currency, which oversees national banks, told Riggs during the fourth quarter that more precautions were necessary to protect the bank company against future losses.

Following the regulators' review, Riggs added $81.9 million to its provision for loan losses. Riggs said it expects most of those losses to come from its commercial real estate loan portfolio. Riggs also charged off $25.9 million in the fourth quarter to recognize losses that already had occurred.

The bank company said loans that are no longer paying interest or are in default totaled $158.8 million in 1990, a dramatic increase from the $4.5 million in troubled loans on the books in 1989.

Allbritton said Riggs is "aggressively dealing with this situation" and hopes to return to profitability in the first quarter of 1991. The bank's annual loss compared with a 1989 profit of $39.4 million ($2.86 a share), and the quarterly loss compared with earnings of $7.5 million (54 cents) in the fourth quarter of 1989. Riggs stock closed yesterday at $8 a share, down 50 cents; the announcement of the losses was made after the stock market closed.

John Hanson Bancorp., parent of John Hanson Savings Bank, said it lost $268,000 in its second quarter, ended Dec. 31, compared with a profit of $44,000 (1 cent a share) in the same period in 1989.

The Maryland savings bank, which operates 15 branches in that state, said it earned $180,000 in the six-month period, compared with a profit of $2.2 million (38 cents) for the comparable period in 1989.

John Hanson Chairman Jack Pollock attributed the performance to a $516,000 addition to the thrift's provision for loan losses.

"The provisions for loan losses were prudent and warranted at this time because the economy and the real estate markets are not turning around as quickly as was hoped," Pollock said.

Columbia First Bank of Arlington, which operates 27 branches in Maryland, Virginia and the District, also saw its profits disappear in the quarter ended Dec. 31, saying provisions for loan losses forced the thrift to lose $2.8 million, compared with a profit of $1.6 million (56 cents) in the same quarter in 1989.

Columbia First added $6.3 million to its provision for loan losses, reflecting the continued deterioration in the regional real estate market.