Riding the crest of some of the best interest rates in recent years, metropolitan Washington real estate professionals are predicting that 1985 will turn out to be the best sales year in recent history here.

In addition, brokers and real estate agents are predicting that January 1986 will be the beginning of another banner year if interest rates remain stable and low.

"Actually, the market here in Northern Virginia is just staggering," said Fred Hetzel, president of the Virginia Association of Realtors and owner of a Loudoun County real estate company.

Adjustable-rate mortgages are now available at 9 percent or less, and fixed-rate loans are hovering around 12 percent, according to people in the industry.

Economists are predicting those low rates will continue, and many real estate industry officials expect rates to slip still further early next year, Hetzel said.

Lillian Dorsman, an agent in the Bethesda office of real estate brokers Shannon & Luchs, said agents and brokers in her office are treating this month's market "as if it were a spring market." Spring is generally the hottest sales time in this area.

Town houses, condominiums and detached houses are selling quickly, brokers said, adding that sales of new condos are outpacing those of older ones.

Last month, Dorsman listed a brick Cape Cod in a sought-after neighborhood off Connecticut Avenue in Chevy Chase and sold it in eight days for $226,000 after receiving three contract offers to buy the house.

Tom Stevens, president of the Northern Virginia Board of Realtors, said sales during the final quarter of this year may not be as strong as those in previous quarters. But most brokers attribute whatever downswing may come to the arrival of the holiday season, which is traditionally a slow sales period.

"I see a very active market in January, especially as long as the stable interest rates continue," he said. "We will see the year take off in January."

"January of 1986 will be the start of a much stronger year than ever before," predicted William Laughlin, president-elect of the Northern Virginia board.

"Consumers are feeling good about buying a house now," he said.

Agents such as Libby Ross of Town & Country Realty in Arlington say they have yet to see the holiday slump others may be talking about. Ross and other agents said there are buyers "ready and waiting" to make a purchase.

Agents complained of low inventories of existing homes throughout the region.

"We are selling properties that have been on the market for many years that have not sold until now," Hetzel explained.

Sales of existing homes apparently are outpacing sales of new homes for a variety of reasons. Builders are finishing the units they sold during the very busy spring and summer markets, said Samuel Finz, chief executive officer of the Northern Virginia Builders Association.

Dwight Schar, head of NVHomes, said some of his Maryland and Virginia developments are almost sold out.

Fairfield Homes reported only a few lots remaining in some subdivisions, but opened a new development known as Raceway Farms off Telegraph Road in southeast Fairfax this week. That development will offer some models that have attracted buyers to the company's sections of the South Run development in southern Fairfax, according to Amy Hawkins of Fairfield Homes.

Gary Garczynski, president of the Northern Virginia Builders Association and head of Signature Communities Corp., said that, although sales of new homes generally have "slowed since October," his own company reported an increase in sales in recent weeks.

Several Maryland builders said a slowdown may be attributable to the surprisingly strong summer market, which attracted more buyers than developers had anticipated.

"Everybody is optimistic that we will have a strong spring," Garczynski said.

Dotty Abt, an agent in Northern Virginia, said the market for new condominiums is "especially strong."

She pointed to projects in the Fair Oaks and Oakton areas where builders have constructed what she considers "quality, low-rise condominiums," which provide private entrances and fireplaces. She said those features are especially attractive to many first-time home buyers.

Abt said she has sold many of these units with prices starting at $70,000.

Abt and other agents said the new-condo market has hurt the resale-condo market for several reasons. New-condo buyers tend to live in them rather than treat them as investment properties. In addition, Abt explained, condo fees in high-rise buildings can run as high as $400 a month in comparison to fees in many of the new structures, where monthly condo charges are less than $100 a month.

The Montgomery County Board of Realtors reported 1,056 housing units sold in October compared with 637 in October 1984.

That represents a 66 percent increase, said Martin Keithline, an official of the Montgomery board. He said year-to-date sales are running 24 percent ahead of last year's figures.

The Northern Virginia Board of Realtors reported a 32 percent increase in sales for this October compared with the same month a year ago. Sales for the year to date are running 23 percent ahead of last year, according to Joseph Hayden, an official of the board.

The Northern Virginia Board includes Fairfax, Arlington, Prince William and Loudoun counties, as well as Alexandria, Fairfax City, Falls Church and other municipalities.

Sales in Prince George's County for the second quarter of 1985 totaled 3,679, a 4.7 percent decrease from 1984's second-quarter total of 3,859, according to statistics supplied by Rufus S. Lusk & Son. "The dollar volume of these sales nevertheless increased from $363 million in 1984 to $434 million in 1985, a gain of 19.6 percent," according to Lusk.