The idea that thousands of Americans are waiting for interest rates to fall so they can buy a house is a myth, Federal Home Loan Bank Board Chairman Richard T. Pratt said yesterday.

Demand for housing is overrated because home ownership is no longer regarded as "a good investment," Pratt told a luncheon audience of the Washington Press Club. "There is a substantial overestimate of housing demand unless things change in a dramatic fashion."

Pratt's views directly contradict forecasts by real estate agents and homebuilders who have said there is an increasing, pent-up demand for housing waiting to be unleashed by plunging mortgage interest rates.

"There is still a tremendous demand. The problem is affordability. . . Once rates come down, you will see a tremendous outpouring of interest," said a National Association of Home Builders official.

The official said a 1979 study by the association showed that the country needed to produce a minimum of 1.8 million new homes and apartments yearly in the 1980s to keep pace with demand.

Instead, housing production has fallen to its lowest levels in 20 years because of high interest rates and high prices. There were 1.3 million starts in 1980, 1.1 million in 1981 and 1.2 million forecast for this year, leaving a 2 million shortfall in just the first three years of the decade, the NAHB official said.

Pratt, a finance professor at the University of Utah and consultant specializing in mortgage lending and utilities before his federal appointment last March, disagrees.

The almost frenzied housing demand of the 1970s was driven largely by buyers who sought housing not only for shelter but primarily as a hedge against inflation, he said. In those boom days, the appreciation on single family homes was running ahead of inflation.

"Financial benefits will be lower in the next 10 years than in the last 10 years," so housing demand will fall, he said after the luncheon, because of the loss of cheap mortgage interest rates that characterized the housing market until October 1980 and the sharp decline in appreciation that struck last year.

In 1977-78, while inflation went up 7.7 percent, the median price of a new single-family house rose by 14.1 percent. But last year, while inflation stood at 10.4 percent, sale prices rose only 6.7 percent, according to the homebuilders association.

"I question his assumption that people primarily bought homes as an investment," said the NAHB official. "People have to live some place. People buy for shelter and tax reasons as well."