Three-fourths of the families in the United States no longer can qualify for a mortgage to buy a medium-priced house, the head of the National Association of Home Builders said yesterday - one reason why housing starts and sales of existing homes show unmistakable signs of slowing.

Housing starts and sales, which once pumped fuel into the fire of inflation, are now beset by the recession syndrome resulting from the anti-inflation campaign. Housing, finance and realty sources agree that mounting costs of borrowing money and lack of buyer confidence to pay rising interest rates are eliminating more and more buyers from the housing market, thus slowing starts and sales.

Vondal Gravlee, a Birmingham builder who heads the 108,000-member NAHB. said yesterday that housing prices now average $53,000-$58,000 nationwide, and a family would have to earn at least $22,000-$24,000 to qualify for a mortgage. Gravlee told a news conference here yesterday that the Department of Housing and Urban Development reports the median income is about $15,000-$16,000 for a family of four.

A one percent increase in mortgage rates "takes 2.9 million families out of the market," he said.

"When you combine all these things, 74 percent of the families cannot qualify for a mortgage of a medium-price home."

Observers are divided on the significance of these indicators. Some see the housing market becoming "normal" while others are expecting the market to tailspin if interest rates remain high and the total economy is chilled.

In the Washington area, where some large-volume home builders still have mortgage money available under the current national level of 10 percent mortgage on resold houses have increased 10 1/2 and 10 3/4 percent in the past week. Money is available for home loans in the District and Virgina, but virtually no commitments are being made by conventional lenders in Maryland, which had a 10 percent usury ceiling on home loans.

Some realty agents insist they are finding some loans here and there, and also arranging loan assumptions or financing by owners wanting to sell. Also, considerably more use is being made of FHA and VA loans, still under a 9 1/2 percent ceiling. But sellers have to pay a premium ($1,500 to $2,000) for a $50,000 FHA or VA mortgage

The bad news in Maryland is that the state legislature is not expected to act until January to raise the usury ceiling, or possibly drop it altogether. But the likelihood is strong that home loans over 10 percent will be legal in the Free State by February.

The gloomiest faces on the housing scene are those of the nation's organized home builders. Vondal Gravlee, president-elect of the National Association of Home Builders, said yesterday that he spent the first three days of this week asking the White House, the Federal Reserve Board and the Federal Home Bank Board: "Do you know what's happening to housing?"

Gravlee said he has been telling government leaders that moves to increase the cost of borrowing money in the fight against inflation have virtually destroyed "what is essentially a strong housing market." Gravlee said builders are facing increasing inventories of unsold houses across the nation and also planning to curtail their production sharply in 1979.

"Instead of the 1.7 million housing starts that many experts are predicting for next year, we see starts dropping to 1.5 miilion," Gravlee said.That would be a decline of nearly 25 percent from the level of 2 million housing starts in both 1977 and 1978.

Gravlee also said reports from 2,174 volume builders showed a decline of 35 percent below the high production levels of 18 prveious months. "But these statistics have not yet shown up in the federal counts," he added gravely.

Martin Poretsky, president of a suburban Maryland builder group, said that buyer interest has tapered off and that plans for 1979 starts are being curtailed.

Larry Breneman, president of Washington Homes Inc., which builds in the moderate price range said: "People are scared by rising money rates and downward economic trends. Many are already borrowed up to their eyeballs."

Poretsky, Breneman and other area builders see little chance of any significant downturn in housing prices because of higher costs of construction loans, now at the 13 percent level and moving upward.

Meanwhile savings and loan associations and other sources of mortgage credit still have money for new loans of current rates. But S&Ls are fearful that holders of the new six-month mortgage money certificates may seek competing higher yields when the first of those $10,000 saving certificates come due in December. However, those rates are keyed to the federal borrowing rate and now yield about 10 percent. It's also a hard choice for lenders to pay that much for some savings and make long-term loans under 11 percent.

In fact, national S&L industry economist Kenneth Thergerson yesterday predicted rates will rise to "11 percent or more" next year. He also expects 1979 housing starts to hit 1.7 million, adding: "The frequently discussed economic recession of 1979 may well occur - but it will occur much later in the year than most forecasters anticipate."

An upbeat note was struck by Ronald Weismiller, who heads an area mortgage banker association: "These are disturbing times and it's tough to make a judgment. But I do know that some big-volume investors are looking for high-yield mortgages in 1979".

Weismiller inferred that investor interest in "locking in high yields for a full year" could indicate mortgage rates might turn down before the end of 1979. "But never again to 9 1/2 percent," he added.

Despite downbeat news and forecasts area brokers and agents are outwardly maintaining traditional confidence. For instance, executive Thomas Shafran of Better Homes Realty said yesterday that two agents recently closed sales on three Northwest Washington houses for a total of $816,000. (The usury ceiling in the District now is 11 percent. There is none in Virginia).

And James D. Burner, a Vienna broker and spokesman for 33 area franchised Red Carpet brokers commented: "Like the Redskins, we professionals will have to work harder in this market. We know that people want to buy houses and we have to help them makes it possible under more difficult conditions."

Prospective home buyers meanwhile are concerned that "prevailing rates" cited in future commitents for mortgages might exceed 11 percent by spring. "I couldn't handle that," one potential buyer said yesterday.