Mortgage interest rates have declined about as much as they are going to, and will probably climb 1 1/2 to 2 points by next summer, officials of the American Mortgage Bankers Association predicted yesterday.

Thus, although now is a "relatively opportune time" to buy or refinance a house, consumers should be prepared to accept "a permanent level of double-digit mortgages through the 1980s," according to Dr. Mark J. Riedy, the association's executive vice president.

The Federal Housing Administration recently lowered the interest rate ceiling on its mortgages to 12 percent while conventional mortgage rates in the Washington area are averaging slightly above 13 percent.

Lenders are already seeing a rush for new or replacement loans, said association president James M. Wooten, creating an increase in applications "unique in my 36 years in the business.

"We have hit a trigger level on interest rates," he said.

He and Riedy expressed concern over the ability of both lenders and federal housing agencies to handle the increased volume of paperwork. Now is "the best time for everyone in the process, but we are creating bottlenecks," Riedy said.

Wooten observed that there is now a "smorgasbord" of mortgage types available to borrowers, with all sorts of rates and maturities. "The cheapest rates are the shortest," he said. ". . . We are seeing a lot of 15-year financing," with rates sometimes running as low as 10 to 11 percent.

He said that interest in adjustable mortgages, which were very popular last year when rates were high, has tapered off. But he added that those who opted for an adjustable note a year ago have been able to "ride it down" as rates have declined and now "don't have to do anything" to enjoy lower payments--though they face increases if rates turn back up.

"It's a guessing game," he said. "Is the wheel of fortune going to stop on 9 percent or 17?"

Mortgage rates will remain high primarily because the traditional source of long-term loan money, the thrift institutions, has been thrown into turmoil by inflation and deregulation and replacement sources have not been found, Riedy said.

As savings and loans move into areas formerly reserved for commercial banks and compete aggressively for funds, they won't be willing or able to provide mortgage money they traditionally have, he said. The thrift industry is going through "a traumatic restructuring" that is not yet complete, and methods of tapping other sources of funds have not been adequately developed.

"There's a lot of money in the country, but it's the wrong kind of money," he said.

On related topics, the two also said:

* The appeal of buying a house as an investment, rather than simply as shelter, has been "pretty well washed out" for years to come. Mortgage costs are permanently higher, Riedy said, adding he doesn't see a return to the rapid increase in prices that took place in the late 1970s.

* Refinancing, while important to those who need it, is absorbing money that would otherwise go to construction, so Riedy foresees only a "moderate" recovery in housing starts next year.

* The Federal Reserve Board is currently pursuing an "accommodative" monetary policy--neither tight nor stimulative--and is likely to continue to do so for the foreseeable future. Riedy said that since the economy is slack, the Fed can afford to allow the money supply to grow without danger of stimulating inflation.

* The current eight-point gap between the inflation rate and the prevailing mortgage rate means that the real cost of credit is very high, but "it is wrong to assume" that this represents large profits for lenders.