There are currently more than 40,000 houses, condominiums and other properties for sale in the Washington area, by far the largest total in recent years and 20 to 40 percent above what used to be considered normal in many jurisdictions.

A survey of area multiple listing services by Long & Foster Real Estate Inc., found 7,085 properties for sale in Montgomery County last week; 4,619 in Prince George's; 2,000 in the District, and 16,540 in Northern Virginia.

When outlying jurisdictions, such as Anne Arundel and Charles counties in Maryland and Prince William, Loudoun and Fauquier counties in Virginia, are included, the total comes to 44,873, the survey found.

Dan Richard, marketing director for Long & Foster, cautioned that the numbers include commercial property and land as well as residences, and that there is a "minimal" amount of overlap caused by brokers' listing the same properties in more than one jurisdiction. It does not include properties for sale by owner.

Richard said he believes the figures are a good index. "The sellers have come back into the market faster than the buyers have," he said, creating what he called "an extraordinary buyer's market."

Spokesmen for most area boards of Realtors agreed. For example, listings at the Northern Virginia Board in May were 30 percent higher than last July, which is when the group started keeping inventory records. In Prince George's, listings as of June 21 were 13 percent over the same period a year ago and 56 percent above the boom times of June 1979.

In Montgomery County, listings currently exceed 7,000, compared to about 6,000 at this time last year; 4,000 in 1981, and 3,300 in 1979. "Would you like to buy a house?" joked one member of the staff. "We're having a two-for-one sale."

Data from the Montgomery board also indicate that about a third of all listings processed by the board have been reported sold this year, whereas the normal figure is about half.

An unusually large number of unsold properties is found across the country as well, according to the National Association of Realtors.

Ken Kerin, an economist with the association, said there is currently a 10-month inventory of unsold residences on the market nationwide. While the NAR has not been keeping this statistic long enough for historical comparison, he said he views the supply as "substantial."

Observers within the industry and outside it attribute the large number of listings to three factors:

* A better selling climate--primarily due to lower interest rates--that has encouraged more potential sellers to test the waters.

* Interest rates that, though lower than last year, are still high enough to disqualify many potential buyers.

* Sellers with unrealistic views of what their properties are worth.

"A lot of sellers are seeing market conditions improving" and putting their houses up for sale, Kerin said. However, the "sales rate has not boomed up high enough to sop up all those properties," he said.

One of the reasons "we haven't seen prices take off" is "all that supply, which has served to dampen" upward pressure, Kerin said.

Paul Fowler, executive vice president of the Prince George's Board of Realtors, said that, ever since the big run-up in interest rates, houses have been remaining on the market longer.

"It takes longer to sell a house," he said.

"The competition is tough," Richard said, adding that his firm is "advising home sellers that homes must be priced within 5 percent of fair market value to sell, and with all the competition, financing is still playing a major role in sales."

He and others in the industry pointed to what Kerin called "the affordability bind" that high prices and high interest rates cause for many buyers. Nationally, despite a big decline in rates beginning last August, a family earning the median income does not qualify to buy a median-priced house.

In addition, they noted, the interest rate decline has apparently bottomed out, at least for the time being, and in recent weeks rates have bobbed up slightly.

The result, they say, is an excellent opportunity for the buyer who can qualify for financing and who is prepared to shop around.

Shopping, they say, is particularly important because there are plenty of properties that remain unsold because the price is too high. One agent on Capitol Hill said his brokerage has a new listing for a house priced, because the owner insists, at more than $200,000, whereas the agent and his colleagues feel the property is unlikely to sell for much more than $180,000.

"Some people are just stubborn," Kerin said. They got used to the rapid appreciation of the late 1970s and are unwilling to take less.

In other cases, sellers are seeking to trade up and feel they must get a certain minimum out of their house in order to buy what they want. "The backlog of buyers we all heard about was also a backlog of sellers," Kerin said.

Nonetheless, Richard said, "a competitively priced property with good financing will sell quickly."

He said Long & Foster is counseling potential sellers to keep the price close to market, to offer any financing they feel they can and to be willing to accept FHA and VA contracts, which sellers sometimes resist because of the delays involved.

He added that other money incentives such as picking up some of the closing costs or kicking in a repair or redecoration allowance can often be crucial.

He said the firm also tried to make the seller understand the risk involved in setting too high a price. Many people "hope a highly motivated buyer will walk through the door" and they will get their price. But if that doesn't happen, the house can languish on the market, acquiring a reputation among agents as one that doesn't move.

When that happens, the seller may ultimately find he has to drop his price below the market to stir up any interest, Richard said.

"It's a gamble," he said.