Home sales in Washington, surrounding Maryland counties and Northern Virginia remained constant or declined during August, reflecting the higher mortgage rates prevailing for most of the month and a seasonal downturn, local realtors say.

But sales continued to outpace last year's recession level, in line with the housing recovery underway since interest rates have dropped.

Realtors said they were unsure how last week's drop in FHA- and VA-insured mortgage rates from 13 1/2 to 13 percent would affect the local housing market; lower mortgages also usually mean that sellers have to pay additional points to lending institutions.

Lenders charge points, each worth 1 percent of the mortgage, essentially to increase returns on mortgages so they can be resold on the secondary securities market. There were indications from an informal National Association of Realtors survey last week that lenders will charge two additional points because of the rate decline.

Many realtors warned that rates must remain stable to maintain a robust housing market. Some even said rate stability is more important to home sales right now than a small drop in mortgage rates, since current rates are tolerable.

The change in mortgage rates Monday was the fourth in less than three months. Commercial mortgage rates have been 14 or 14 1/4 percent lately.

Patrick Kane, regional vice president of Hugh T. Peck Properties Inc. in Wheaton, said sales in August have kept pace with results posted in July, and are about 50 percent ahead of last year.

Kane said, however, that higher rates at the beginning of August forced many first-time buyers out of the market, especially in the $60,000 to $90,000 price range.

William McLane, regional comptroller with Century 21, said even though his company, which sells in the District, Virginia, Maryland and lower Delaware, experienced a 3 percent downturn in business in August, he said the decline could be attributed to seasonal trends.

In 1982, Century 21 business went down by 10 percent from July to August, he said.

Ray Chappell, president of Merrill Lynch Realty in Washington, said that having his own mortgage financing company enabled his firm to package attractive financing to buyers and maintain sales volume in August, despite higher mortgage rates.

Northern Virginia home sellers, bearing the brunt of a marked decrease in sales over the past few weeks, are facing a highly competitive "buyer's market" that has depressed home prices, realtors said.

"Sales are 70 percent of what they were in June and July," said John Adams of Avis, Adams & Associates in Vienna. Hardest hit by the July and August hikes in mortgage rates, when FHA and VA rates went from 12 to 13 1/2 percent, were homes in the $90,000 to $120,000 price range, he said.

Nationally, preliminary figures from the National Association of Realtors show a 4.4 percent decline in sales of existing homes, from a seasonally adjusted annual rate of 2.94 million units in June to 2.81 million units in July.

The decline is basically tied to a rise in mortgage rates from June through August, said Frank Katusak, vice president for economics and research. He noted that June home sales figures had been only 1 percent more than figures for May, indicating a slowdown in the housing recovery because of higher mortgage rates.

A spokeswoman for the Mortgage Bankers Association said an informal survey had shown that applications for mortgages had fallen off about 20 percent after the FHA and VA rates went up to 13 1/2 percent Aug. 1.

Sales of new single-family homes and town houses in the Washington area fell 19 percent from June to July, and sales of condominium units dropped 5 percent, according to Debbie Rosenstein of Housing Data Reports Inc., a local marketing research firm.

In addition to the dampening effects of higher interest rates in the first half of the month, Rosenstein said a low inventory depressed sales levels in the area.

Housing starts dropped 0.6 percent in July for the second month in a row, the Commerce Department reported last week, falling from a seasonally adjusted annual rate of 1.75 million units in June to 1.74 million units. Housing starts in May were 1.79 million units, a 3 1/2-year high.

The dip in interest rates last week left potential buyers, already shakey, even more confused, realtors said.

"At Sunday open houses, we were telling potential buyers the rate was 13 1/2. Do we call them back and tell them it's 13 percent?" asked Gayle Warman, manager of Long and Foster's in Alexandria.

Richard Lanham, president of Beltway Homes Inc., said: "Stability is the name of the game. The public needs to know that whatever rates are prevailing will continue to prevail."

"As long as people can look at budget and know that mortgage rate is going to stay there," they'll be happy, said Kane of Peck Properties. "Where the caution comes in is when you have to tell them if FHA changes the rate next week, you'll still qualify for the loan but your payments will be more."

Interest rate charged is the rate prevailing when the sale is closed, not when the contract is signed.

Phil DeWitt, a spokesman for the Prince William County Board of Realtors, said, "Some of the rate changes are more confusing to sellers than to buyers," because they affect the number of points sellers must pay lenders.

Realtors do say that the earlier, higher FHA and VA rates lowered points for sellers. Kane said points fell from 6 or 6 1/2 to 2 1/2; Chappell of Merrill Lynch Realty said points declined from 5 1/2 or 6 to 2 or 3.

Some realtors think the latest mortgage rate change was a well-intentioned attempt to reviving the housing market, which has appeared to be slumping recently, but they question the timing and volatility of the changes.