Savings and loan associations, banks and other institutional lenders charged an average interest rate of 16 3/4 percent for conventional, new-home loans committed early last month, setting a new national record, the Federal Home Loan Bank Board reported yesterday. The average rate in the Washington area was only slightly lower at 16.71 percent.

The national average, up from 13.37 percent a year earlier, reflected a decrease in deposits and the lenders' increased costs of borrowing, the bank board noted. The loans were committed for newly built single-family houses, loans that normally have 75 percent loan-to-price ratios and 25-year maturity terms.

Rates for conventional loans closed early in June also reached a new high of 14.81 percent nationally and 14.83 in this area.

The survey does not cover sales of used housing -- traditionally about three-fourths of the housing market activity -- and fails to reflect the burgeoning use of so-called "creative financing," where sellers help finance their buyers. Such arrangements are believed by some observers to facilitate at least half the house sales today, and at below-market rates.

The survey responses of about 1,100 to 1,200 lenders indicated a reduced volume of lending and "a wider disparity than usual in the rates being reported," bank board economist Stephen Zabrenski observed.