Interest rates on 30-year fixed-rate mortgages, which had dropped to below 9 percent last spring, are expected to rise above 11 percent by the end of the year, while interest rates on adjustable mortgages are expected to increase more than 1 percentage point to above 9 percent, key housing officials said this week.

Economists and housing experts said the rise is due to concerns about inflation, the tightening of the money supply by the Federal Reserve and a weakening dollar. Several other interest rates that lenders use as guidelines to help set their mortgage rates have also increased during the past week.

The Federal Reserve raised the discount rate from 5.5 to 6 percent to try to help stabilize the dollar, and rates on three-month Treasury bill securities rose from 6.19 percent to 6.45 percent, their highest level since March 1986.

"There's a fundamental perception in the marketplace that we're seeing an end of the three-year decline in interest rates," said Tim Howard, senior vice president of economics, strategic planning and financial analysis for the Federal National Mortgage Association, known as Fannie Mae. "They can go nowhere but up."

Lyle E. Gramley, chief economist for the Mortgage Bankers Association of America, said the trend toward higher mortgage rates could continue until mid-1988 as long as there is a threat of higher inflation.

Robert Van Order, chief economist for the Federal Home Loan Mortgage Corp., known as Freddie Mac, said home buyers should move quickly if they are thinking about buying.

"There's no reason to keep waiting for the rates to fall because they won't," he said. "We're not expecting long-term rates to go up much more, but it's hard to predict changes."

As a result of the interest rate rise for fixed-rate mortgages, there has been a recent decline in the number of refinancing applications, which had reached record numbers last year, and a bigger spread between the number of home buyers choosing fixed-rate and adjustable-rate loans. During last spring, nine out of 10 home buyers opted for fixed-rate mortgages, but by summer that share dropped to seven out of 10 mortgages, according to the National Association of Realtors summer mortgage finance survey.

Lenders have been attracting borrowers to the one-year adjustable-rate mortgages, on which the interest rate can be changed yearly, by keeping the adjustable rates lower than for 30-year fixed-rate loans. There currently is more than a three-percentage-point spread between fixed-rate mortgages at nearly 11 percent and adjustable-rate loans at about 7 3/4 percent, the largest spread in two years, according to Robert K. Heady, publisher of the Bank Rate Monitor, a weekly newsletter that analyzes interest rates of banks and thrifts throughout the country.

"As fixed-rate mortgages rise, adjustable-rate mortgages are remaining affordable," said Mark Obrinsky, an economist with the United States League of Savings Institutions. "But with the rise in the discount and Treasury bill rates, we're going to see a rise in the adjustable rate. So, they won't be as low as they have been all year."

Since March 18, mortgage rates have been steadily on the rise, Heady said. At that point, the rate on a 30-year fixed-rate mortgage was 9.19 percent and the one-year adjustable-rate mortgage was at 7.47 percent. In the Bank Rate Monitor's survey of interest rates this week, mortgage rates on a 30-year fixed-rate loan rose from 10.64 percent last week to 10.95 percent. The one-year adjustable rate rose from 7.65 percent last week to 7.73 percent.

The rise in mortgage rates from 9.19 percent to 10.64 percent on a 30-year fixed-rate $80,000 mortgage would cost a person an additional $85 a month, Heady said. If the rates rise to 11 percent, a person would pay an additional $107, and at 11 1/4 percent a borrower would pay an additional $122 compared to mortgage rates last March.

Economists and housing officials urged borrowers to look at not only the stated mortgage rate that lenders are offering, but also at the number of points being charged. A point is equal to 1 percent of the total loan amount. Every additional discount point is equivalent to about one-eighth of a percentage point increase for the overall rate.

"Lenders are more likely to raise the points in the immediate term," said Martin Regalia, chief economist for the National Council of Savings Institutions. "It's a psychological thing. People who are obtaining mortgages are less sensitive to point increases than to rate increases."

In one Washington area example, First Washington Mortgage this week was charging borrowers who obtain a 30-year fixed-rate loan for 90 percent of the total value of the house an interest rate of 10 1/2 percent with 3 points, up from 10 percent and 3 1/2 points last week, which would be a net increase over the life of the loan. "If rates stabilize in the 11 percent range, the Washington market will still be hot, but some other areas will be hurt by the rate increase," said Kent W. Colton, executive vice president of the National Association of Home Builders. "Some buyers will no longer qualify for loans, and others will switch to adjustable-rate mortgages. There will be some slowing down in sales, however." Washington Post Staff Writer Ann Mariano contributed to this report.