Much of the rest of the United States has seen prices of single-family residences soar in the past three years, but not Long Island. It has suffered through a uniquely depressed real estate market that is the mirror image of the housing boom elsewhere.

But Long Island, while it has its special problems, may portend what could soon happen in other parts of the country.

The dramatic tightening of credit in the past week by the Federal Reserve board is expected to quickly translate into higher mortgage rates, which may finally slow the feverish upward spiral in housing costs that has been taking place - if not lead to some actual declines.

New York State has the lowest usury ceiling in the country - 8 1/2 percent. And the impact of higher interest rates has been felt here for more than eight months. It has taken the form of extremely scarce mortgage money, and required down payments of 35 to 50 percent in many cases.

The savings banks, which are the main source of mortgages, say they are continuing to lend, though they are increasingly limiting loans to long-term depositors.

But industry sources say the savings banks find whatever excuses they can not to make loans.

The truth is that all of the savings banks are out of the market," said one real estate official. - Any one who would lend at 8 1/2 percent when the prime rate is at 10 1/2 percent is a fool, and I don't really blame them."

Federally charted commercial banks do not have to observe the ceiling, but are subject to federal limits of 1 percent above the Fed's discount rate.

But last week, the Fed increased the discount rate by a full percentage point to 9 1/2 percent as part of the program to defend the dollar. This would allow commercial banks like Citibank and Chase Manhattan to raise the mortgage rate as high as 10 1/2 percent. None of the banks has announced an increase yet, however.

Citibank, the largest commercial bank in New York, moved Monday to raise its own mortgage rate from 9.5 percent to 10 percent, plus a one quarter percent origination fee. The new level is a record for New York state. But Citibank said it raised the rate so it could continue to make home loans.

Chase Manhattan and the National Bank of North America, the other federally chartered bank that makes housing loans, are expected to follow suit shortly.

Inexpensive housing has been harder hit than higher-priced houses located in the northern shore "gold coast" communities of Long Island that are within easy commuting distance of New York City; the wealthy seem both better able to afford housing prices and find it easier to get financing.

And resales are doing, somewhat better than new construction, according to housing officials.

Prices of newly built houses on Long Island are averaging $46,500 and have gone up only about one percent a year for the past three years, according to Thomas Junor, executive vice president of the Long Island Builders' Institute.

And in the case of some particularly depressed communities like Brookhaven - the largest city on Long Island and once the center of the island's building boom - there actually has been a depreciation in purchase prices as some of the highest property taxes in the country have combined with hard-to-come-by mortgages to limit buyers for the many homes to be found on the market.

"It is very common to find a home-owner paying $3,000 in property taxes for a home originally purchased for $40,000 that is now selling for $35,000," said Phillip Giaramita, deputy town supervisor for Brookhaven.

And Frank Anzini, a local realtor, said it is typical for homeowners to be paying property taxes between 7 and 8 percent of the original purchase price.

And while many homeowners might be able to afford a $350 monthly mortgage payment, they cannot contend with property taxes of $250 a month on top of that.

One realtor said that foreclosures in Suffolk County, where Brookhaven is located, have exceeded new housing starts so far this year.

The problems of Brookhaven and other parts of the New York State real estate market really go back to 1974, the last time interest rates soared to double-digit levels and mortgage money became scarce.

Because of the low usury ceiling, New York was particulary hit by the credit crunch and, along with the double, whammy of a particularly deep recessionary impact on the local economy, the real estate market recovered much more slowly than it did in other areas.

The ceiling also could guarantee that the current depressed market lasts longer.

Brookhaven, which is famous for the nuclear research laboratory located there, is an extreme case of a vicious combination of factors that feed on themselves to make the situation worse.

Located 60 miles east of New York City, Brookhaven, at 320 square miles, is the largest township on Long Island. For many years, people referred to Brookhaven as the "boom town," because in the past 25 years its population went from 50,000 to 350,000. Most of the increase took place in the late 1960s and early 1970s.

The economy of the city was based on the construction industry that built the houses that attracted people to the commuter "bedroom" community. By the early 1970s, the city government was taking in $3 million a year, which was ample to run the city and provide local services.

But the 1974 recession hit the housing construction industry very hard, and far fewer firms came back than had been building before the recession.

The renewed decline in housing starts this year has decreased housing permit fees and mortgage tax receipts from $3 million to only $1 million. As a result, despite a real reduction in the budget from $16.1 million to $15 million next year, Brookhaven has had to resort to a sharp increase in the property tax rate to make up the rest of the difference.

The rate per $100 of assessed valuation has been increased from $1.76 to $2.33, a jump of nearly one-third.

And property assessment rolls which used to increase by about $20 million a year, are growing by only $11 million this year, with all but $3 million of that attributable to a Long Island Lighting Co. nuclear power plant.

The only building in the community in the past two years was the result of a special mortgage subsidy program from the federal Farmers Home Administration. But earlier this year, new guidelines promulgated by the Agriculture Department eliminated Brookhaven from the program, which was responsible for about 1,000 new homes annually.

As a result, new home construction this year is down 70 percent from the already depressed levels of a year ago, according to deputy supervisor Giaramata.

"We're caught in a vicious cycle," Giaramata said. "we have a real problem attracting businesses because of our tax rate. And as long as we can't attract businesses, we can't give home-owners any relief."

This in turn has depressed housing prices.

It is impossible to generalize from Brookhaven's uniquely different situation to other areas in the country.

But it does raise in a red flag of warning about what can happen to other communities once housing values, which represent a larger and larger part of what many families think they are worth and which have provided what has appeared to be the best hedge against inflation, begin to deteriorate.

"If home prices flatten out and decline against the extraordinarily heavy costs of home ownership these days, we could have a real problem," said economist Alan Greenspan, former chairman of the Council of Economic Advisers in the Ford administration.

The ratio of scheduled mortgage repayments and other consumer installment debt payments as a percentage of cash disposable income has accelerated sharply in the last three quarters until it now stands above 25 percent for the first time, Greenspan said.

This ratio was below 20 percent throughout the 1950s, and hit a peak of 23 percent in the 1960s, before falling back again to about 20 percent. It started climbing again in 1975.

"The average household today is becoming more and more like the farmer who is land rich and cash poor," he noted. "The average family is turning out to be house rich and cash poor. And implicit in that is that there will be further borrowing in the credit markets against the equity value of homes."

"There can be decline in housing values," the well-known business eonomist added. "But the implications are very deep-seated" because of the extended repayment situation of many homeowners, he said. "And this may turn out to be one of the major problems facing the U.S. economy in the next three years."