WHEN SHE'D like a change from the city and up-to-date design at her Watergate apartment, Frankie Welch can holiday with history in Alexandria's landmark Gilpin House.

Welch doesn't charge rent to mice in the cellar or pigeons on the roof, but she manages to make money out of every other nook and cranny. Gilpin House is a god example of making preservation pay.

She rents the first floor to a bookstore, the third and fourth floors to Time-Life as a corporate pied-a-terre, the flounder wing to young couple -- and her own second floor to corporations too posh to entertain in hotels.Sometimes the organization wants the rooms for the night, sometimes just for a luncheon. One couple rented it for a wedding reception and for their wedding night. The fee is $300 a day.

Welch is not only a landlady. She is also the owner of dress shops, which include her "Frankie" dress design among the couturier fashions. Recently, she has become more involved in her custom silkscreen fabic business. She usually is a walking advertisement for her silkscreen fabric company, wearing a different Frankie scarf around her neck every day. She's an impressive size of a woman, and her clothes are carefully chosen to complement.

Welch has a deep voice and a deep Rome, Ga., accent. She's friendly in a Great Dane sort of way that threatens to bowl people over. But she's so helpful and nice that she's managed to ingratiate herself with first families from the Lyndon Johnsons to the Carters. In 1968, her famous customers made her famous herself -- that was the year she did scarf designs for both the Republican and the Democratic campaigns.

Today she has shops dispensing clothes and her own fabric designs in Alexandria, Washington and Atlanta, with more planned in Florida. She says Mrs. Carter pops over from the White House to look at clothes in her conveniently located shop at Pennsylvania Avenue and 17th Street.

Welch is a good example of the Georgia girl who learns early to speak soft and think hard. Southern women, many of whom lost husbands and fathers in the War for Southern Independence, left a longtime legacy to their great granddaughters of how to get things done fast while adopting an air of "I've got all the time in the world for you, honey."

Welch hands out scarves to everybody who comes to a party at one of her many locations (and that's everybody from the wife of a former president of the United States to women who're presidents of companies themselves). She is gives scarves away with as much abandon as if they were African violet cuttings. Her eight-inch module scarves are repeated to make fabric by the yard used for wall coverings and curtains. Her custom scarves have been commissioned by the Marine Corps, Exxon and tons of banks -- she figures some 800 commercial companies and 2,000 political organizations, high schools, weddings, museums and such.

At Gilpin House, a silkscreen of a fabric design hangs over the living room fireplace. And of course the napkins are Frankie's. The rest of the rooms rug, and exuberant Art Nouveau mirror, a Russian samovar, needlepoint chairs and all sorts of interesting things to distract the bored corporate visitor.

But in the rear apartment, the flounder wing, with its 18th century corner cupboard, there's literally one of each of the scarves Welch has made. They're framed, sometimes in four units, sometimes singly, to cover the staircase wall. The Cherokee alphabet fabric covers the sofa and more Frankie-designed fabric is draped over the tables and the pillows. There's never any doubt as to who decorated the house.

(Flounder houses, as they are called in Alexandria, are flat as the fish, with windows only on one side and a steep shed roof, pitched in one direction. They were the first units of houses built in Alexandria and were usually small cheap houses, 1 1/2 stories, built quickly in accordance with the terms of the lot sales. They were designed so they could be joined to an adjacent house on the other side and larger, more impressive section added in the front.)

Frankie Welch started silk screening in school, on paper. She studied art and fashion, among other things, at Furman University and the University of Wisconsin. "From my window, I used to see Frank Lloyd Wright walk down the street. His clothes were wonderful -- a gray herringbone suit, a beige and chocolate brown cape. He dressed perfectly for his size and image." b

In 1966, Virginia Rusk, wife of the secreatry of state, asked her to make a gift for the State Department to give foreign dignitaries. Welch recalled her silk screen studies and started silkscreening scarves. In tribute to Dean Rusk who came from Georgia and her own Cherokee acestor, Welch made a scarf using the Cherokee alphabet. From there she's gone on to make scarves for everything from the the Folger Library to McDonald's.

Welch hit town in 1963, when her late husband, historian William Welch, came here to work with Rep. Henderson Lanham of Georgia. Her husband was later congressional liaison for the Veterans Administration.

Welch taught clothing and design at the University of Maryland, acted as a personal consultant to women on their wardrobes and eventually, for three years at the Congressional Club, taught wives of senators and representatives how to dress. She met many of her later famous customers that way.

She decided then to set up a dress shop. The Welches first bought the Duvall House in Alexandria, a circa 1750 house in rather dilapidated repair, but with a rich history as a tavern where George Washington dined and as a bank where he kept his money. She opened a shop on the first floor. She, her husband and their two daughters lived on the second, with the flounder section in the back as the family room.

With her husband's skills as a historian and her talent for publicity, she soon turned the restored house into a tourist attraction. When women she'd met at congressional parties came to shop, for privacy they tried on dresses in Welch's own living room.

Welch early on invited former dancer Betty Ford, then a Congressional wife, to model her clothes. They were members of the same Alexandria church. Later on, Mrs. Ford give her "Frankie-designed" dress to the Smithsonian. Welch knew Lady Bird Johnson through Bess Abell and Liz Carpenter of Mrs. Johnson's staff. For her she did a "Discover America" scarf, and was rewarded by being asked to do a fashion show in the White House.

Welch bought Gilpin House after her husband died. Col. George Gilpin was a business partner of George Washington and drew the 1799 man of Alexandria. When he built the front part of his 3 1/2-story brick house in 1798, he attached it to an older flounder wing. Even then, it had a commercial establishment on the first floor and a separate entrance to the upper stories.

When the house was restored in 1930, by Wes Wilson, who had worked on Williamsburg, paneling was brought in from Woodstock Inn for the entrance hall and mantels from La Plata, Md. Wilson had his antique shop on the first floor, where the book-store is now. "He said he would only sell it if I would buy it," said Welch. "And two months after I did buy it, he died. He loved the house so he wanted to be sure it was in good hands."

Welch, having used up the available spaces in Gilpin House in Alexandria, has recently rented an apartment in Watergate so she'll have another place to hang not only her hat, but her fabrics (see the companion story on Page £1.)