The Washington area housing market, a key indicator of the region's economic health, is down in the dumps at a time of year when sales and construction normally leap forward.

The number of condominiums standing idle throughout the area with no buyers in sight has risen a startling 88 percent from last year to 5,100, according to Renay Regardie, president of Housing Data Reports Inc., which does marketing research on new housing in the area. Regardie noted, though, that this increase was partly explained by growth in condominium construction.

New single-family houses idle on the market are up 20 percent over last year to 3,900, Regardie said.This was due in part to heavy cancellations of purchases at the end of last year.

But the most important explanation of the area's housing doldrums is the high interest rate on mortgages -- now averaging 15.5 percent, with no improvement on the horizon. "I look for more of the same for the rest of the year," said Thomas J. Owen, chairman of Perpetual Federal Savings and Loan Association, "a very flat and stagnant year."

New construction of single-family dwellings throughout the area for the first three months of 1981 is projected at 22 percent behind last year, according to Michael Sumichrast, chief economist of the National Association of Home Builders. Sumichrast projected an even sharper decline in new construction for the next three months.

"Some improvement is expected late in the year, as interest rates fall off a little," Sumichrast said. "But, any way you look at it, it's not going to be a good year."

The nationwide housing picture, Sumichrast said, is even gloomier. Across the country, 340,000 new houses have not been sold, mainly because of the high interest rates. Sumichrast noted that 115,000 of these have been completed and, of those "one-third are more than a year old and two-thirds are more than six months old."

Generally, new housing stock in the Washington area is bought up more quickly, according to Regardie. "We have nothing on the market for as long as a year," she said.

She added that sales last month of new, single-family houses and town houses in the metropolitan area were up 41 percent over January and 6 percent behind February 1980. Condominium apartment sales this February were 44 percent ahead of January and 78 percent ahead of last year, "but this is because a couple of large projects came on the market" during the month, Regardie said.

Nationally, according to Sumichrast, less than 6 percent of all households are able to qualify for a mortgage on the median-priced house, which now costs $65,000. In order to qualify for a $60,000, 30-year mortgage, a family must have an annual income of $47,000, he said.

In the Washington area, where the median price of a house is $100,800 -- fifth highest in the nation -- realtor Gordon Carey of Merrill Lynch Realty/Chris Inc., said families that only two years ago could qualify for loans to buy houses in the $125,000-$250,000 range find the doors of savings and loan instituites slammed in their faces.

"As recently as 1979," Carey said, "two-income families earning $40,000-$50-000 a year could qualify for houses like that. Now, their income has to be almost 50 percent higher."

According to Carey and other realtors, sales of new houses and resales of older structures are most vigorous at the two ends of the price scale. Houses below $125,000, which qualify either for Federal Housing Administration or Veterans Administration loans, and those over $250,000, where buyers have no need for outside financing, are selling comparatively well.

"The day of the million-dollar house is here in Washington," said Brian Logan of Long and Foster Realtors. "Most of them go to diplomatic missions, but quite a few go to private individuals. This is a phenomenon that hit California first and had us all gasping. But whatever happens in California comes to Washington two years later. People who buy at that level don't finance -- they pay cash."

For those shopping below the million-dollar mark, though, financing is the foremost impediment to home buying. While realtors questioned tended to maintain that money was readily available -- "if you're prepared to pay the high rates" -- Perpetual's Owen said this was not the case.

"Money is very, very tight," he said. "The rates on the average are over 15.5 percent. Most of us aren't looking for loans because of the heavy flow of funds out to money markets and similar investments. We've moved to severely restrict our loans and we're lending only to our existing customers."

Owen added that earlier expectations of a drop in interest rates below the teens no longer seemed likely. "I don't even foresee a drop to 13 percent by the end of the year," he said.

"There's a shrinking pool of people who can afford these high rates," said Elizabeth Twigg of Brenneman Associates Inc., a prominent condominium development and sales firm. Twigg noted the formula for calculating the size of a mortgage that a would-be buyer could afford to carry has altered markedly since interest rates jumped in October 1979. "It used to be 2.5 times your gross annual income," she said, "but now it's 1.8."

These fiscal hardships have put a crimp in the life styles of buyers who must have a house at this time for one reason or another. "They're moving farther out of town or they're buying town houses or condominiums or, if they insist on a single-family home, it's a smaller house," said Carey of Merrill Lynch.

Carey, like many realtors, said that people who could qualify for loans at high interest rates were getting their money. "We find that the buying public is accepting the high rates," he said. "People are saying, 'Forget the rates. Tell me what my monthly payments will be. Can I afford it? Can I qualify?'''

Carey and others on the sales side of the industry say they are optimistic about the coming spring season, but all conceded that they had little of substance on which to pin their outlook. "As a company, we're running about 55 percent ahead of last year," Carey said. "But the overall market (in resales) is down about 32 percent."