A middle-income home buyer this year is spending a third of his income on monthly mortgage payments -- nearly double what he spent seven years ago.

Contrary to predictions of economists, housing costs have continued to outsrip gains in income, making it more difficult for homeowners to afford the houses they buy.

The median monthly mortgage payment this year is estimated at $488, which is 33 percent of the national median income of $1,486 a month, according to Data Resources Inc., a major economic forecaster.

In 1972, mortgage payments amounted to 16 percent of income.

In the Washington area, estimated costs of other homeowner expenses such as real estate taxes, insurance and utilities would consume another 13 percent of the median income of $17,842 a year, meaning the built-in costs of owning a home would amount to almost half of a person's income.

But people continue to buy, jumping in to avoid being priced out of the market and expecting their houses to appreciate rapidly and justify their actions.

"You can't postpone it because prices keep going up, and incomes aren't going up at the same rate," said Freddie Biddle, an accountant at Common Cause.

Biddle and her husband, who works at the Commerce Department, bought a $40,000 house in Columbia Heights, the area around 14th Street and Columbia Road NW, just over a year ago. A real estate agent recently told them their investment is already worth $80,000 to $90,000, she said.

To afford the former rooming house that Biddle said was "in need of massive renovation," the couple has sacrificed vacations and is living in the debris of continuing repairs. Only one of the building's three floors is finished.

Biddle estimated between 40 and 45 percent of the couple's net income is being plowed into the house.

"Really, we're both just working to build and restore this house," she said. "You have to have a place to live.

"We looked around for a long time, but the prices were constantly rising."

"It's total inflationary psychology," said Dennis J. Jacobe, an economist for the U.S. League of Savings Assocations and a student of the national trend.

But in addition to the classic inflationary spiral of demand driving up prices and rising prices drawing more people to the market, at least two other factors are contributing to the current climate.

The World War II baby boom generation that swelled the nation's school population in the last decade has now entered the housing market, adding to the inflationary pressure. And couples in that age group -- now between 18 and 33 years old -- frequently have two careers, with strong incentives to reduce their income taxes by investing in houses because mortgage interest and real estate taxes are deductible.

They have surged into the market in unexpected numbers.

And interest rates, now at historic highs, also have contributed to rising mortgage payments.

Jacobe pointed out in a study last year that buyers have been able to stay in the housing market by going deeper into debt. But he warned there were limits, and those limits would be reached first in overheated housing markets like Washington.

Another expert, Kenneth Kerin, economic vice president of the National Association of Realtors, said, "They do buy, but I think they're making severe sacrifices in their budgets to stay in the housing market.

"With another notch in the interest rate and higher fuel prices, a lot of people are going to be shaken out."

Many would have been forced out of the market already by old rules of thumb used by lending institutions to determine would-be buyers' eligibility for mortgages. One of those rules was that a buyer should not pay more than 25 percent of his or her income in mortgage payments.

But one agent said that rule has been relaxed to permit people to spend from 28 to 33 percent of their incomes for housing.

Data Resources had predicted in 1977 that, as 1980 approached, housing would become more affordable. That prediction relied in part on historic trends. From 1955 to 1970, median family income grew more rapidly than the median sales price of a new, single-family home.

The predictions, however, did not materialize, and Data Resources now concedes it misjudged the market.

"I think two to three years ago most of us didn't feel affordability would be too severe a problem," Kerin said. "We didn't feel that housing prices would outstrip income as fast as they have because period most of us looked at the long-term trends. The relationships didn't change that much in the long time period."

In past periods of rapid inflation in housing costs, "what happened was the credit crunch," said Jacobe, the savings association economist. "Whether the buyer was willing to pay the price or not, he couldn't get any money." But Jacobe said no such force has intervened yet nationally in the current booming market.

In the District, however, mortgage money from savings and loan associations began drying up in April and is still tight, said Bruce Bryan, executive vice president of the Metropolitan Washington Savings and Loan League. That followed a decline in deposits in savings accounts in those institutions. Mortgage money for homes in Maryland and Virginia is still generally available, he said.

Banks have been willing to relax guidelines and continue making housing loans, in part because "the appreciation of the homes themselves makes them sounder to finance," according to an official at the Federal Home Loan Bank Board.

Several economists also noted that people are getting larger, better-equipped houses for the higher prices they pay.

"While the median sales price is rising faster than median family income has risen over the years, the median price per square foot has not," said Robert Gough, who did the original Data Resources study. "What people are opting for is a more embellished house."

The larger-house phenomenon is fallout from the notion of housing as a good investment, several economists said. Instead of putting money into other investments, young, two-paycheck couples are buying as much house as they can. To some extent, that type of investing -- plunging every penny into housing -- may contribute to the capital shortage facing the nation, the U.S. League of Savings Associations study noted.

"Because of inflation in housing prices, many people have a great deal more housing equity than they had before," said Jacobe, who authored that study. He called this the "wealth effect" which may balance some of the sacrifices made to buy a home.

Buyers may have to plan on putting a larger than traditional percentage of their paychecks into house payments in the foreseeable future, said Kerin of the National Association of Realtors. "Now we're getting to the point where it's normal to spend a third of your income on housing," a figure which wouldn't be striking in other parts of the world such as Western Europe and Japan, he said.

"In our country we've been spoiled by housing that is very affordable," he said.