Economists looking at mortgage interest rates expect they will remain high or increase slightly this fall as the result of the administration's credit-tightening policy. At the same time, the economists anticipate a significant decline in demand in the next few quarters.

At this week's auction by the Federal Home Loan Mortgage Corp. of six-month forward commitments, yields rose substantially. "The large increase in interest rates in the financial markets has probably caused some mortgage sellers to reevaluate the likelihood of an early peak in interest rates," said Dolores Lynn, an economist for the Mortgage Corp. "The fear of continued high or higher interest rates may have prompted the increase in yields."

The average yield, which rose from 10.977 in July to 11,388 this week, represents mortgage lenders' estimates of what their rates will be six months hence. The July figure had shown a significant dip from those of the preceding months.

Kenneth Biederman, chief economist for the Federal Home Loan Bank Board, predicted mortgage rates would remain high or go higher, reaching 12 percent in certain areas, including Washington. The cost of funds to savings and loan associations plus slower growth in deposits, as customers put their funds into higher paying investments, combine to keep interest rates high. Inflation in house prices also are making the dollar volume of mortgages equal to earlier in the year, even though unit sales were off 7 percent last month.

Kenneth Kerin, vice president for economics and research of the National Association of Realtors, said an earlier predicted drop of 1/4 to 1/2 percent in mortgage rates by the end of the year would not occur, but that rates would remain about where they are now. Unit sales of new and existing houses have fallen about 10 percent this year and another 5 percent decline is expected by the winter.

Robert J. Sheehan, associate chief economist of the National Association of Home Builders, said the awareness that the administration and the Federal Reserve would keep interest rates high forced the trade organization to revise downward its 1980 forecast for housing starts.

Next year NAHB looks for 1.5 million new houses under construction, a 25 percent decline from 1978's peak of 2 million.

Advance Mortgage Corp., an affiliate of Citicorp, noted "a sharp dropoff in new home sales in nearly all local markets, beginning about midyear," President Robert J. Mylod said that mortgage rates over 11 percent "didn't seem to matter in last year's boom," but they do now. He noted that the 1 1/4 to 1 1/2 percent rate increase of the past 12 months, on top of a nearly 15 percent increase in average new house prices, translated into a 25 percent total increase in monthly payments, while personal income has increased just over 10 percent.

As a consequence, high loan rates are shrinking the number of people who can afford to buy homes. Mylod said sales in Chicago, hit by winter storms, were down 40 percent during the first half of this year, despite rates lower than in most markets.

In the Washington area, two surveyers of mortgage rates say lenders report sales are off slightly. They differ, however, on the prospect for interest rates this fall.