The current decline in interest rates should continue at least until the end of the year, and while there is likely to be an upturn next year, 1985 still will turn out to be a relatively good year for housing, according to a panel of economists assembled this week by the National Association of Home Builders.

The recent slowdown in growth of the gross national product has allowed the Federal Reserve to ease up on the money supply, resulting in the easing of rates since their July highs, they said.

"I think what's happened here is this last decline in rates over the last two or three months has bought you an extra good year, an extra good spring and summer of 1985," Leonard Santow of the New York economic consulting firm of Griggs & Santow told the builders.

But overshadowing the short-term optimism was serious concern over the long-term effects of the large federal budget deficits, which most of the panelists agreed show no signs of going away. There was considerable skepticism about Reagan administration claims that economic growth will bring the deficits down.

Under Santow's projections, the government will need roughly a 10 percent increase in receipts next year to stay even, and the next recession the deficit will leap to the $250 billion to $275 billion range.

Lawrence Chimerine of Chase Econometrics and Michael Sumichrast of the Home Builders agreed that, as Chimerine put it, "We'll probably scrape through," but also that, as Sumichrast said, "The risk on the down side is enormous."

Much of this risk stems from the fact that the economy is now in effect perched atop an enormous pile of foreign capital that has flowed into the United States in response to its high interest rates and to economic problems abroad.

This extra money, coupled with lower-than-normal corporate borrowing, has enabled the capital markets to supply both government and the private sector -- and notably housing, which Sumichrast said consumes about 30 percent of the capital supply -- without a serious crunch.

"I have no idea how long this inflow can continue," Santow said, but there is probably a limit on how much dollar-denominated investment most foreigners want in their portfolios.

In any case, should the dollar weaken significantly and the inflow become an outflow, the economy would be in for a shock.

Chimerine said average real growth of 2 1/2 percent over the next two years "in my judgment is what we're going to get in the United States until we permanently bring interest rates down and until the dollar comes down in an orderly way. For both of them, I think we will need a sharp reduction in future deficits.

"And the worst scenario," he added, "is if the dollar collapses before we reduce deficits. We're financing these deficits with money from overseas. We need it badly. . . . But if that stops coming in before we reduce deficits, we could see very, very sizable increases in interest rates."

Neither he nor the others said they saw any great likelihood of such a collapse -- Santow expects the dollar to continue strong for at least another six months to a year -- but neither were they optimistic about the chances of a major deficit reduction.

With President Reagan apparently committed to his policy of growing out of the deficit, Santow said it may be 1988 before anything serious is done.

But amid the others' concern about the deficits, Robert Barbera, chief economist of E. F. Hutton & Co., noted that they are highly stimulative -- "one foot on the accelerator" -- and that given the current low inflation rate he believes there is room for the Fed to ease monetary policy even further.

"If you tell me what housing is going to do, I'll tell you what interest rates are going to do, rather than the reverse," he said. He believes the Federal Reserve "has a mandate for disinflationary economic expansion" and that all it can do when it sees things moving too fast is put interest rates up.

Thus, housing has become "the marginal sector" that "has to absorb the pressure when the economy is moving along too quickly because fiscal stimulus is pushing those other sectors."

This means that when housing starts reach the 2 million level as they did early this year, an interest rate rise will be right behind because the Fed will see the starts rate and put its foot on the brake.

But the pattern of the year illustrates that the nation has been able to have a lot of real growth and low inflation, Barbera said, adding that "those low inflation numbers mean we can afford somewhat more growth than people now anticipate."