Fulfilling the American dream of homeowner-ship is out of the question for many young Americans caught in the worst affordability crisis in more than a decade, real estate experts said.

"Housing affordability conditions for first-time buyers, compared to those for the overall home-buying public, have deteriorated to the worst point in 13 years, and we don't think the situation will remedy itself," said Nestor R. Weigand, president of the National Assocation of Realtors based in Washington, D.C.The NAR and other industry groups are calling on government to help ease the burden on first-time buyers.

According to the Commerce Department Census Bureau, the national homeownership rate has decreased to 64.0 percent in 1987 from its 1980 peak of 65.6 percent. The drop has been most acute among younger people aged 25 to 29, where a 7.4 percent fall-off in homeownership to a rate of 35.9 percent occurred between 1980 and 1987.

According to the NAR, median sales prices of existing single-family homes stood at $87,900 on a national basis in the fourth quarter of 1988, up 3.4 percent from the year before.

But the uptrend in prices is more conspicuous in such cities as San Francisco, Los Angeles, Philadelphia and Chicago, which posted respective year-to-year quarterly increases of 29.6, 24.6, 13.0 and 10.8 percent.

As a way to measure first-time buyers' home-purchasing power, NAR comes up with an index every quarter that reflects the gap between income and house prices.

The first-time homebuyer affordability index is calculated based on a buyer making a down payment of 10 percent of the purchase price and obtaining the remainder as a loan at the prevailing interest rate.

The 1988 October-December index stood at 78.9, which means typical first-time homebuyers had 78.9 percent of the income needed to qualify for a mortgage on a typically priced starter home.

The number was well below the composite housing affordability index of 117.7, which measured affordability conditions during the same period for all purchasers of existing homes.

John A. Tuccillo, NAR's chief economist, said the continued rise in mortgage interest rates likely will cause the housing situation for first-timers to worsen throughout 1989.

"As rates go up, we expect to see the afford-ability gap become wider," he said.

Often the biggest hurdle facing those who want to buy is the accumulation of down payment.

"Accumulation of down is becoming tougher among first timers largely because rent itself is high," said James C. Murray, senior vice president of Weichert Realtors in Morristown, N.J.

"In the early 1970s, rents were averaging about 23 percent to 25 percent of one's income. They are up to 40 percent now, making it more and more difficult for people to save up the required down payment," he explained.

Despite all the difficulties, owning a home is still a big part of the "American dream."

A 1988 poll conducted by the National Housing Institute based in Orange, N.J., showed two-thirds of those under the age of 35 who do not own a home consider buying one an important goal, and 17 percent regard it as their most important personal goal.

The hunger for homeownership prompts people to commute unprecedented distances to their workplace in order to get into a home they can afford.

For example, Eastern Pennsylvania - some 65 miles from New York - is now considered part of the New York home market and is thriving as such, said Murray.

Buyers are clearly sacrificing location to achieve ownership, real estate experts said.

A March survey conducted by Weichert Real-tors showed nearly 60 percent of renters polled are willing to double their commuting time in order to own a home.

While some banks, such as Dollar Dry Dock in New York, offer so-called zero point mortgages that merge closing costs into the amount financed to eliminate initial cash requirements, real estate industry representatives are critical of the government for not coming to the aid of first-time buyers.

"We are not calling for more layers of government bureaucracy to solve the affordability problems faced by first-time homebuyers," said NAR President Weigand, "but a national housing policy that places homeownership at the high economic and social priority it deserves."

Congress is expected to soon consider legislation that would allow first-time homebuyers to use funds in Individual Retirement Accounts for down payments on first homes, without having to pay a stiff withdrawal penalty.

Another bill designed to increase the amount of Federal Housing Administration-backed loans also is being studied.

"The current FHA limit is so low that it is not enough to buy anything with," Murray said. "FHA was a great, great vehicle 15 years ago. But not now."