A 45 percent year-to-year gain in sales of previously owned homes in Montgomery County tops August results in the metropolitan Washington area, according to data released this week by a listing service and by several industry organizations.

Sales in Northern Virginia were up by one-third, and D.C. sales also increased.

As sales continue to soar, prices are increasing in some parts of the region by as much as 10 to 20 percent.

For example, prices in the District are up 10 percent over a year ago, according to statistics compiled by Rufus Lusk & Sons Inc., a real estate information service. And prices in some of the close-in suburban markets such as Great Falls, McLean and Bethesda are as much as 20 percent higher than a year ago, according to industry sources.

The increase in prices and a continuation of stable interest rates in the 11 to 11.5 percent range for fixed-rate mortgages are prompting more people to buy homes, real estate brokers explained.

Many people apparently are buying now rather than later because they are worried about increasing prices and the ever-present possibility of a jump in interest rates. Many who qualify to buy at today's rates might not meet a lender's requirements if interest rates jump above 12 percent, according to agents and lenders.

Bob Coursey, marketing services manager for the Maryland region of Ryan Homes, reported "exceptionally strong sales. We had the strongest July and August in the 14 years we have been in the Washington region." He said that the growth is "virtually all attributable to financing that is now available."

Coursey and other new-home builders predicted a leveling off -- but no major decrease -- of sales in the fall. Builders predicted that traditional dramatic hikes in fall sales won't materialize because the summer slump that usually comes never hit.

Industry sources also pointed to several trends in the current market, including:

*Continuing interest in purchases by first-time buyers in spite of stricter scrutiny by lenders.

*Increasing popularity of the Arlington area as a bedroom community for the District and Tysons Corner.

*Continuing strong sales in the western suburbs of Fairfax, Prince William and Loudoun counties, where housing dollars buy far more than in closer-in neighborhoods.

*Increasing popularity of town houses and of new and resale homes in the Laurel and Bowie areas of Prince George's County.

*Strong sales along the Route 270 corridor in Montgomery County despite highly publicized traffic problems in the area.

*The emergence of the Silver Spring area and other neighborhoods near Metrorail stations in the Maryland and Virginia suburbs as strong resale markets and areas where developers are seeking land for town-house construction.

*The emergence of the Oakton area of Fairfax as a strong source of housing for people employed in the Fairfax Center, Tysons Corner and Fairfax City areas.

A spokesman for the Northern Virginia Builders Association said sales slumped slightly during the Labor Day weekend but "are now picking up, as people are seriously looking into making a move."

In addition, homes in areas inside the Capital Beltway are selling fast, agents reported.

"I am not kidding, I have people waiting in line, but I don't have much to show them. I hit the computer and get only five or six listings," said an agent with McLean's Laughlin Realty. She was referring to listings of available housing units in the Northern Virginia Board of Realtors Multiple Listing Service.

Agents in the Oakton and Vienna area also reported low numbers of listings and said housing in the $150,000 to $200,000 range is extremely hard to find.

Sales are especially brisk in new-home developments in the Centreville area, where several major developments are under construction, according to agents at several projects.

In addition, sales volumes reported generally do not reflect new-home sales because they often are not listed in MLS operations in Northern Virginia or Montgomery and Prince George's counties. Home builders report strong sales, and many say they are taking contracts on properties they won't be able to deliver until 1986.

As in the resale market, builders are reporting price increases, partly because land is becoming increasingly hard to find and more expensive.

"Builders are running out of buildable parcels of land," according to Tony Ahuja , an official of the Northern Virginia builders group.

According to Lusk's figures, the average price of a house in the District hit about $125,302 during the second quarter. Although those numbers include the sale of four properties at more than $1 million each, "You can still buy a house in a decent neighborhood in the District for under $100,000," Rufus Lusk III said. The average price of a house in the District has increased by 10 percent since the summer of 1984, according to statistics supplied by Lusk.

The Northern Virginia Board of Realtors reported only a 3 percent hike in sales prices across the board from August 1984 through last month, but board President Tom Stevens said prices in some areas leaped as much as 20 percent. The board's report covers Prince William, Loudoun, Fairfax, Fauquier and Arlington counties, and Vienna, Alexandria, Falls Church, Fairfax City and other municipalities inside the listed counties.

Stevens, an official of Shannon & Luchs, said his company's office in McLean reported 20 percent year-to-year increases in house prices in August. He called those increases "shocking," with average prices jumping "from $112,000 to $199,000 in a year."

Those figures may reflect more than simple increases in value, several agents said. McLean agents said those figures may be attributable to more stability in the homes selling in the $100,000 price range than in the homes in the $200,000-and-above range.

The 5,000-member Montgomery County Board of Realtors reported August resales of 1,069 units, compared with 736 a year ago.

In some areas of Montgomery and Fairfax counties, homes in several new developments are hitting the market priced well over $500,000, and several are advertised as having starting prices near $1 million.

In Prince George's County, sales continue to be "good," according to Rusty Lobban, head of the Prince George's Board of Realtors' multiple listing operation. Sales are ahead of last year at 4,188 units this year compared with 3,692 at the end of August 1984.

Condominium sales in Prince George's County in August were down to 72 from last August's sales of 78 units. In the District, condo sales jumped from 412 in the first quarter to 667 in the second quarter.