The Washington area has become a magnet for out-of-town merchants.

They have come from all over the country, lured by the demographics. The Washington metropolitan area has the third highest per-capita income in the country - $8,454 - according to the latest figures from the Department of Commerce. Moreover, huge population increases are expected in nearby Virginia counties, and households are getting smaller, giving family members more money to spend. And there are a high level of education and a high proportion of white collar workers in both Montgomery and Fairfax counties, according to one store's careful surveys.

But the clincher, says Philip Miller, president of Neiman Marcus, one of this year's entries in the retailing race, is that there is little manufacturing in this area. "Most people are working where they have to wear coats and ties and dresses for work. Not overalls, as they do for factories."

In the last week, three major retailers opened stores in the area: each with specific notions about Washingtonians; and each with a different idea of itself and how it can serve Washington customers. The styles of the openings reflected these differences.

There were hugh baskets of bread shipped in from San Francisco and set as decorations on the bars. A chic-ed version of Fisherman's Wharf displayed bay shrimp - and Dungenes shrimp was placed among the accessories. I. Magnin, the california-based chain with a high-fashion emphasis for both men and women, had finally opened its doors in White Flint.

There was nothing quiet about this store's opening. It was touted in the Vandenburg room of the Capitol in March, followed by a capsule fashion show at the Madison Hotel. And last night I. Magnin threw a benefit party for 1,500 to celebrate its opening and to raise close to $50,000 for the American Cancer Society. (For those who missed it, there will be a "second coming" for the benefit of The Washington Home next month.)

Why Washington, I. Magnin's president, Norman Wechsler, was asked as he checked displays and chatted with department managers before the store opened yesterday. "Why not?" he shot back, obviously used to the question. "It's a world center, it's cosmopolitan, and it's affluent," he said firmly, emphasizing each point equally.

("I don't think we'd do well in Miami Beach," he added.)

Another factor was "available space," Wechsler said. Bloomingdale's, like Magnin's a Federated store, had shared studies on the area with Magnin's. The results looked promising, "and so there seemed no reason not to march right in."

"There's no store like us in Montgomery County," said Wechsler, indicating some of his competition with expressions he does not want repeated. "Just say that there is no one in Montgomery County with the same level of merchandise, in terms of fashion and quality."

In addition to all the studies, Wechsler knows his competition. Until three years ago he was Magnin's competition - as president of Saks Fifth Avenue.

"This isn't a city of fashion mavens," Wechsler said, "but from what I see and know, you want good fashion here, particularly quality." He also is counting on Washingtonians' propensity for travel, as well as their "infernational origins," believing that "Europeans really are more conscious of fashion."

This is I. Magnin's 24th store, and the first so far east. But Wechsler isn't worried about translating the so-called California life style and clothing for the Washington woman.

"We're not Hollywood, y'know," he said. "We're San Francisco, where they wear navy and pin-stripe suits," as well as Marin County and the peninsula, where life is "casual, easy, not structured, free-flowing," - words he says apply to the clothes here, too.

Magnin's buyers see the same designers as many other stores do, but their clothes are chosen together, not as designers see them, but as we would from our closet - with a sweater from one person, a skirt from another." Sonja Caproni, the store's fashion director, added, "the mix is essential."

But the California look is very hard to define, and there is no way to be sure how Washington will accept it. Wechsler admitted he's nervous. He kept referring to California as a "feeling, unstructured, ad lib," and cut off a suggestion that it has to do with colors or specific types of clothes.

"That's why Washington is so important," he said. "If we can transplant the feeling of California to the East Coast, the world is our oyster."

Magnin's plans to move on to Fairfax County. "And in three years," Wechsler hopes, "maybe downtown Washington." Bands and Balloons

"We're not the catered cocktail party crowd - we're strictly coffee and donuts," said Irwin Greenberg, president of Hess's department store, which opened its sixteenth store in Frederick, Md., last week.

So it was high school bands and helium balloons, searchlights and skydivers, and of course coffee and danish when the store, based in Allentown, Pa., and known for innovative promotions opened last week. Hess's usually sponsors a marathon on opening day, but this time the store had difficulty locating a club. Besides, the weather wasn't very good.

Hess's, the first store to have Rudi Gernreich's topless bathing suits (1964), and in 1973 the first store to carry Russian designer fashions (including some with see-through tops), thinks of Frederick as a "bedroom community for Washington" and strictly "middle America," not very different from the other cities in which they are located.

"Everything that is good for Allentown is good for Frederick, or whatever place we are," said Phil Berman, the store's chairman of the board.

So why the $5,000 silver-plated grasshipper in the china department, or the $2,000 Spanish ceramic chess set in gifts? "People go to a department store for entertainment, to see what's new. To look at it, admire it, sometimes even with disbelief, and sometimes even buy it," said Berman.

"Middle America loves to shop in a store that has these things, even if they are just going to buy an Arrow shirt, an Evan-Picone suit or a $10 gift certificate," Greenberg added.

So what's new in Hess's these days? How about an Acu-Ring for $9.95, created by a Chinese-trained medical doctor to control one's diet by putting pressure on the ear? According to Berman, Hess's is the first store in the world to have them.

Of course, it might have cut out the coffee and donuts. A 'Laid-Back' Line

Last Saturday, the day August Max opened for business in White Flint, a young woman from Philadelphia was parading in front of a mirror admiring the new fall coat she had selected after trying on several, and modeling it for her boy friend.

Nonchalantly he dug into his pocket, handed the woman a small box and said, "Why don't you see how this goes with the coat?" It was an engagement ring.

Kathleen Jellison, vice president of August Max, who was on hand when the chain's eleventh store opened here Saturday, swears the story is true. While it's never happened before, she said, "it's not impossible it will happen again. We're that kind of a "laid-back" store.

If their opening was also "laid-back" - no more than some 1940s picano music and a live mannequin in the window while store executives, including president Judy Stewart, stood by - there was nothing unplanned about the store's arrival.

Besides researching the dempgraphics, Jellison shopped 150 stores in the area to measure the competition and hung out in malls and in Georgetown to see what Washington women like to wear.

Her conclusion? "The Washington woman relies on designer labels and isn't very secure about what she chooses," said Jellison (For example, she pointed to women who wear Calvin Klein tops with matching skirts.) She spotted a fondness for the classic here, "without much imagination."

"It's a big town for Ultrasuede [synthetic fabric] and louis Vuitton, floral skirts and short-sleeve shirts. But the key accessories are missing that give the Washington woman any kind of an individual look," Jellison concluded.

The best reason anyone can give for the name August Max is that most of the stores have opened in August. The Washington store, no exception, zeroes in on the current fashions, including unconstructed jackets, huge sweaters, tapered pants and broad-shouldered coats, with fabric and color mix possibilities displayed on oak and copper shelves and racks.

Jellison believes the Washington customer is ripe for some suggestions, both through attractive store displays and sales help. So ripe, in fact that the store is projecting several additional outlets in the area, including one on Connecticut Avenue later this year.

But before that, Jellison is off for the next August Max opening in Chicago. "That customer is a bit more urban that Washington," said Jellison. "She has a much better array of stores to shop in there than the customer does here. Somebody did the groundwork for us out there." Top of the Curve

But if there is such endless marketing potential in the Washington area, what ever happened to Mark Cross and Mario Valentino, or even Kann's and lansburgh's? And why are there frequent rumors about stores not doing well, why is there a low fashion profile in this town?

Sometimes it is undoubtedly the fault of retailers who misjudge the customer and don't mind the store. And there is always the competition of nearby New York.

But it is possible that the same demographies that have lured the stores here are misleading them as well. Washington has a hugh upper-middle income group. And many of the stores are counting on a population swell at the top of the income curve, which they are courting with high-fashion, high-price merchandise.

However, that group may not be so large at the top; and maybe for a large portion of those who was there, fashion does not have the high priority Washingtonians have customarily given to real estate, or educating the children, or travel.

But some of the offerings at the new stores and old are going to make the current fashions hard to resist. And that's what they are counting on.