The coming months here and on the mainland are uncertain for anyone planning to buy or sell a house.

Unmistakably but subtly, those who attended the annual convention of the National Association of Realtors here this week had to recognize signs for caution ahead. These 28,000 conventioneers, many of whom make exceptionally high incomes as brokers and agents for property transfers, know better than anyone that the past two years have seen high levels of sales.

From Seattle to Miami and San Diego to Washington, prices of most reasonably attractive houses have increased 15 to 30 percent a year. In Honolulu, condominium apartments are being sold for more than $125 a square foot - more expensive than luxury units in Washington.

While this phenomenon in the nation's housing may thrill sellers - particularly when they don't have to buy another house, or if they are moving to areas where housing prices aren't as high - it discourages buyers who want an affordable place to live.

At the same time, people who don't own houses, but who have seen values double in five or six years, are encouraged to buy.

Today's buyer faces mortgage interest rates that have soared from slightly more than 9 percent to more than 10 percent in a year. Up until now, many buyers have been willing to pay asking prices because mortgages were available in those states without usury ceilings.

But escalating housing prices have made monthly mortgage payments of $600 and more common for hundreds of thousands of Americans who have bought houses priced over $75,000.

Mainly, the crisis looms ahead. Already some home builders are reporting that sales have been impeded because prospective buyers couldn't sell their houses for the high prices they thought they should be able to get and were unable to trade up.

So far the money market has been more inhibiting for banks lending to customers and banks borrowing from the Federal Reserve. But now mortgage-sensitive realtors are concerned that the supply of loanable funds from savings and loan associations might be sharply lessened if the S&Ls decide in the next 30 to 60 days that they cannot afford to pay savers more than 9 percent on six-month mortgage certificates. S&Ls feel they must have a spread of at least 1 1/2 percent on money they borrow and then loan out to home-buyers.

S&Ls retained many millions in saving this year by offering savers with accounts of $10,000 or more the opportunity to make 1/4 percent above the federal borrowing rate on a six-month basis. Next month, the first of the money market certificates will expire.

There is mounting concern that individual S&Ls will not be able to offer the opportunity for reinvestment at rates exceeding 9 percent.

For the prospective home buyer or seller, this means a cloud on mortgage money availability over the next six months as earlier money market certificates run their course. Those certificates have been credited by housing observers with saving the market from fund disintermediation and a mortgage crunch earlier this year.But mounting rates paid for savings, even on a short-term basis, have to portend even high mortgage interest rate requirements.

Is American ready to handle 11 percent mortgages?Some experts think the rate is not important if buyers can handle the payments.

One alternative, and it has some drawbacks, is the current 9 1/2 mortgage financing available through FHA and VA loans, which have a government-set ceiling. But every ceiling and good deal has its price.

If the government-backed loan is $50,000, the seller the builder of a new house or owner of an existing dwelling - now must pay at least 4 percent of the amount or $2,000, for the privilege of getting a 9 1/2 percent loan for the buyer. When that amount gets to 5 or 6 percent, as it is expected to do if the FHA-VA ceiling remains stable, then more sellers will think twice and possibly veto use of this good selling tool.

And that's really the gist of the current housing market. Everyone has said or heard someone say that housing prices have gotten so high that they'll have to come down some time. That time may be at hand. But the come-down is not expected to be dramatic, at least not at this reading of the real estate industry.

The consensus at the realtor convention indicates strongly that housing sales will slow down in the next few months. It is also believed that mortgage interest rates will rise another 1/2 percent but will come down in six to nine months, mostly because of the decreased demand for funds.

Realtors also maintain that the public appetitie for residential purchases remains stronger than ever and will continue so into the 1980's because the number of new households will continue to increase.

Finally, real estate and housing have been going so strong in the past three years that some sort of a slowdown has been in order. This one is expected to be less traumatic than the big shakeout of 1973-74. That's the good news. The bad news: It's not a promise - it's only a forecast.