More than 20,000 new dwellings will be for sale in the area this year, and couples with two incomes and single persons will be buying a healthy share of them. An increasingly potent factor in the nation's housing market-place, these and other buyers have helped make the last 16 months extremely active for sales of new and used houses and condominium units in the Washington area.

But despite the added interest of these first-time buyers, some apprehension is emerging that rising interest rates and higher house prices are combining to cool off the market. Mortgage money is nearly 3/4 percent higher than it was a year ago, when most rates were under 9 percent, and house prices are generally up from 15 percent to 20 percent over last spring.

House sales dropped precipitously in the early summer of 1973, when mortgage credit was both expensive and scarce. Today, while credit isn't scarce, it is expensive, averaging 9 1/2 percent.

Additionally, there's the elements of increased competition among builders. More than 100 new housing developments have opened for sales here this year.

Kenneth Murphy, new-house sales director of the Long & Foster real estate firm says that, while gross sales are up this year the average number of sales per project are down. His judgment is that the market will become even more competitive in coming months. He maintains that "it has become a buyers' market and the value-conscious public will be taking a close look" before deciding.

Builders and realtors here agree that buyers of new houses continue to look for energy-conserving features ranging from heat pumps to added insulation and storm windows.

Renee Regardie of Housing Data Reports Inc. says that an "energy package" is standard now rather than an option for new homes here.

The two-story, detached house with basement still is this market's most popular house. Because of rising costs, the typical house is a bit smaller, but four bedrooms, two and a half baths, two-car garage and family room are still standard.

Although the fireplace is still the top-selected option, builders say buyers are increasingly willing to pay extra for bay windows, skylights or "greenhouse windows," usually in the kitchen. Larger, fancier bathrooms are also in favor among young buyers.

Because it requires no down payment and has a loan rate below the conventional market, the VA loan available to armed forces veterans has been used increasingly in recent years. That's because more home builders and sellers, particularly in the lower end of the market, have decided to pay the necessary price to provide those loans.

"Fifty percent of our sales now are VA," said Spencer Stauffer, marketing chief for the Miller & Smith home-buildign firm.

The sales successes of area condominium town house and apartment projects have added to the competition in the house sales market here. High-rise condominiums such as Skyline Plaza, the Rotonda, Watergate at Landmark and others are also active. Recently the 154-unit Westbridge condominium on the edge of Georgetown was sold out within weeks of a public offering that coincided with the start of construction.

One woman with two daughters said she is selling her detached house in Springfield, Va., and buying a town house in Fairfax's Burke Centre because she has less time for home maintenance and yard work these days. Energy-saving features, confidence in the builder and a warranty plan helped her decide on the house, she said.

She said she is giving up a big yard without a twinge and is getting relatively the same amount of interior living space in her new place.

"I also took some extra-cost options, such as a fireplace, a finished family room with a powder room nearly, upgraded carpet and title and hardwood floors in the living and dining oroms," she said.

She said she watches the progress of construction on her town house on weekends and revels in the trees that are being saved.

A homeowner since 1965, when she assumed an original VA loan at 4 1/2 percent interest, she said she "suddenly realized that I had little tax advantage with a small mortgage."

The sole support of her family since her former husband died last year, she is buying a house that is about $5,000 higher in price than her Springfield home. But she is holding the mortgage on the sale of her house and will get "sufficient monthly income to handle the higher payments ($550 a month) on the new house."