Nearly two million American families are paying rent this summer rather than owning their own house or condominium, as they'd fully expected they would by mid-1983.

By the end of the coming decade, 10 million American families who otherwise would be homeowners--by all historical tests of income, education and social status--will be stuck as apartment renters, many of them permanently.

Those are previously unpublished projections from the federal government's top census expert on housing, and they've got him deeply worried.

Arthur F. Young--America's "Dr. Numbers" for residential real estate--directs all the U.S. Census Bureau's studies on housing. They range from once-a-decade, comprehensive counts of hundreds of millions of people to month-by-month analyses of how many Americans rent real estate, and how many own their homes.

It's the latter that has Young so concerned at the moment. He is seeing changes in traditional demographic patterns emerging this year that he feels have profound implications for the country.

He feels that they could begin influencing the resale value of your home before the end of this decade, for example. They could determine whether your children will be homeowners. They could have a direct effect on where you live, whom you live with, how you vote--and whether you vote at all.

What Young is seeing emerge, month after month, is a long-term downward trend in the rate of home ownership in the United States. For the first time since the Great Depression, this rate is remaining flat or declining, despite the moderation in mortgage rates experienced after 1982's historic highs.

The national home ownership rate as of July sat a full two percentage points below the 65.8 percent it registered in the fall of 1980. That translates out to between 1.5 million and 2 million households that are now renting, rather than owning, as they would have under the conditions of 1980.

Young believes that, if economic and public-policy trends continue on their present paths through the 1980s, "There is no question that the home ownership percentage will keep dropping," he said. How far is anyone's guess, but any prolonged decline--reversing four decades of steady, upward movement--"has frightening social and political implications for this country that I don't think many people understand yet," Young asserted.

Here are some of the statistical signs that get little or no publicity, but that concern census experts:

After nearly 15 years in which the number of new household formations in the United States averaged 1.5 million to 1.6 million a year--no matter what the economy did--the number took an unprecedented drop last year to just 391,000. New household formations--whether from marriages, divorce or children moving out of the family nest--are the fuel on which home building, home resales and apartment rentals run. Last year's precipitous drop is a portent that the depth of demand for housing may not be anywhere near as great as many economists believed.

The vacancy rate for single-family homes nationwide is sitting at a historically high level of 1 1/2 percent while the vacancy rate for rental units is rising. "You've got tremendous numbers of unsold, unoccupied units sitting out there right now," Young observed. "People can't sell them, they can't rent them, and they've already moved somewhere else."

Let the trend keep growing, and "you're going to see too many housing units chasing too few households who can afford to pay for them" later in this decade--eventually triggering declines in property values, he warned.

Young minces no words about the forces depressing home ownership: The national deregulation of financial institutions two years ago, plus heavy federal deficits, have raised the cost of mortgage money beyond the reach of millions of people who consider themselves future homeowners.

"We're telling these people, in effect, 'You can't be part of the system, you can't have a piece of property, however small it may be,' " Young said.

"That, in my opinion, is social dynamite in a society like ours. We are on the verge of breaking a fundamental social compact among economic classes that we've had for decades: Work hard, be part of the capitalist system, and one day you or your children will become property owners."

What remedies does Young offer? He emphasizes that he's not in the business of advocating housing programs, but he thinks Congress and the Reagan administration need to look hard, soon, at broadening incentives for first-time buyers, such as tax-exempt, cut-rate mortgage bonds and special tax-exempt savings accounts for new buyers.

"You've got to open the door wider at the low end of the system, or everyone in the system--high and low--is going to have trouble," Young said.