Sales of new and existing homes, town houses and condominiums are soaring in the Washington metropolitan area as home buyers take full advantage of continuing surprisingly low interest rates, real estate professionals say.

Existing homes for sale are being gobbled up and inventories depleted. New-home builders are selling products they won't be able to deliver until next year, they said this week.

Agents, brokers and builders throughout the region are predicting the strongest summer sales in recent years, particularly in Virginia.

"The market is booming from Roanoke to Tidewater, from Tidewater to Loudoun County," according to Fred Hetzel, president of the Virginia Association of Realtors and head of his own Loudoun County real estate firm.

Sales in Fairfax are outpacing sales in Montgomery County and in the District, according to statistics supplied by Rufus S. Lusk & Son Inc.

Hetzel and other agents said they are surprised by continuing low interest rates, which this week dropped to about 12 percent for conventional fixed-rate loans and 11 1/2 percent for VA and FHA loans.

Buyers are surprised by low interest rates, according to Bob Banner, head of Bethesda-based BNR Realty, which does business throughout the Washington metropolitan area. His company recently did an informal survey of potential first-time buyers' attitudes about interest rates.

"They perceived interest rates to be much higher than they are. They perceived 30-year fixed-rate mortgages to be at 13 percent" when the rate is closer to 11 or 12 percent depending on the lender, Banner said.

"There are still a lot of people waiting for the rates to come down. They have come down," he said.

Lenders also have been surprised at continuing low rates, according to Kevin Connelly of Perpetual American Mortgage Co., an Annandale-based firm operating throughout the Washington area.

"We are cautious that sooner or later the rates will bottom out. Rates seem to come down painfully slow, but when they bottom out, the rates really shoot back up in a short period of time," Connelly said.

He said lenders also are being "cautious about originating a lot of loans at low rates," because they may have problems when they try to sell those loans in the secondary market if rates turn up.

F. Gary Garczynski, president of the Northern Virginia Builders Association, said current high sales volumes have created a dilemma for builders.

"Most of us have sold out way in front. We have already sold houses with delivery dates running into next year. People see a window of opportunity for taking advantage of interest rates now," he said. But those rates may not exist when builders finish the houses they have sold and buyers can go to settlement.

"Do we go out and buy [blocks of money to lend buyers] . . . to guarantee interest rates?" asked Garczynski.

"I think that is what is going to happen. Builders are going to be willing to do this," he said.

Signature Communities, Garczynski's own company, reported a 100 percent increase in sales in the last three weeks.

Based on housing sales for the month of May, which were a whopping 17 percent ahead of figures for May 1984, the Northern Virginia Board of Realtors said 1985 "may turn out to be a record year."

Susan Matlick of the Suburban Maryland Building Industry Association said sales are good in Montgomery, Prince George's, Charles and St. Mary's counties.

She said Prince George's County is seeing good traffic from buyers and attracting builders that have traditionally shied away from that county. The Milton Co. and Design Tech, a development firm that has specialized in "in-fill" construction in Montgomery County, are now moving into the Prince George's market, Matlick said.

Sales in Montgomery County "continue to be real good. We have been pleasantly surprised that the low interest rates have continued. We hope they can continue for at least the rest of the year," said Hans Nessler of the Montgomery County Board of Realtors.

Nessler said "higher-priced homes [in the $250,000 to $500,000 range] are moving extremely well" in the Bethesda and Potomac areas. The same holds true in the McLean-Great Falls-Vienna area of Fairfax County.

Agents in the Vienna/McLean area complained that they have more buyers than listings for homes in the $150,000 to the $250,000 range.

New-home builders across Virginia may have been caught off guard by the continuation of low rates, Hetzel said.

Rates had risen gradually through March and then leveled off in April. Since then they have been dropping fast, and builders who had already sold units in advance may not have been ready for the continuing sales boom.

There has been little speculative building of detached or attached houses in recent years in the Washington area because too many builders were overstocked when interest rates soared in 1980. Several builders this week said they wish they now had finished products on sites where grading is just beginning.

People who could not qualify for high interest loans are returning to the housing market, those who were caught in rental units are buying when their leases expire and those who were waiting to buy a move-up house are buying, agents said.

Sales are brisk in close-in neighborhoods along I-270 in Montgomery County, Nessler said.

Convenience to Tysons Corner and the District has become a factor for buying a house in Arlington, where taxes and sales prices generally are a little lower than in close-in Fairfax neighborhoods convenient to Tysons.

"Anything within 20 to 30 minutes to Tysons is selling well," said Renee Miller, of Century 21-Royal Properties in McLean.

"People have always heard that now is the time to buy," she said. Now is a good time to sell and buy, she said.

"There is a great demand for resale housing. A seller can be firmer in the terms he takes in selling his house," Miller said.

"It is not a buyers' market. It is not a sellers' market. It is the ultimate market," said William Laughlin, a Northern Virginia Board of Realtors officer.

"What I have seen happening is a semblance of parity, a close proximity between the buyer and the seller. Prices are not escalating rapidly and interest rates are low. When you get people dealing on the same level, it is a better atmosphere for everybody," he said.

Hetzel said sales in Loudoun are great. "We're running out of listings. This is not unique to Loudoun. It is happening all over the state," he said.

Sales reported by Lusk in Fairfax for the first quarter of 1985 indicate an increase of 14 percent in the number of sales of residential and commercial units over the same period of 1984.

"Dollar volume is up almost 20 percent," noted Rufus S. Lusk III.

In the District, Lusk reports sales are down for the first quarter, but brokers said home sales are picking up and are brisk now that rates have dropped.

Sales in the District, according to Lusk, totaled 1,953 during the first quarter compared with 2,193 in the same period in 1984. However, dollar volumes for this year hit $447 million, well over the $382 million figure reported for the first quarter of 1984, Lusk said.

In spite of reports that condominiums are not selling well across the nation, several agents said condos are selling well in the Washington area. But there are so many units for sale that the market apparently is glutted.

Low interest rates and a lack of available properties make condo investing attractive to investors who cannot afford to buy single-family homes, according to Banner. First-time investors can take advantage of single-digit interest rates to buy $60,000 to $80,000 units that are close to the District or other high-employment centers, brokers in Bethesda and McLean said.

Sales reported by the Northern Virginia Board of Realtors include Falls Church, Arlington, Alexandria and Loudoun, Prince William and Fairfax counties.

Sales in May hit 2,251 compared with 1,927 for May, 1984, an increase of 17 percent, according to NVBR. Sales for this year are up 14 percent over last year's figures.

In Montgomery, May sales were up 10 percent with a 19 percent increase in dollar figures, according to the Montgomery Board of Realtors.

Sales in Prince George's County came to 626 in May compared with 603 in May 1984.

Almost every source contacted for statistical information said its figures may differ from those reported by others. Statistics produced by boards of Realtors, for example, usually do not include new-home sales. Lusk figures contain sales that are settled and recorded with local governments. Real estate boards generally report sales based on contracts reported by their membership.