THE HOUSE is all over Washington -- a self-assertive front porch with white columns or brick arches stuck on a flat front Georgian brick (often yellow) facade. The two honest stories have a third floor with dormers that looks as though it were squeezed out of the red tiled roof, and there's a basement that started out as a foundation and kept on growing.

The front porch (if it hasn't been removed because of chronic leaks) extends across the front of the house. Often the back of the house has a porch on each of the major floors -- a back porch and above it a sleeping porch. Some are rowhouses, others are detached. Some have center halls, others a big reception room.

Hundreds were built on every street of Washington during the 1920s. L.E.Breuninger & Sons, builders-realtors, were responsible for some 2,000 houses, many of this design. You see them northwest, southeast, all around the town. I lived in one on 16th Street for five years -- the first time I had lived that long in any house. We loved its high ceilings, generous rooms and comfortable informal feeling.

In Washington, the want ads call them "colonials," which seems a hilarious misnomer. I suppose the one-story porches could have been faintly suggested by Thomas Jefferson's Roman Revival porticos.

A similar house was built all over the country at about the same time, even some of Frank Lloyd Wright's early houses seem to be sophisticated, slicked up versions. They have some kinship with bungalows, though they are much grander -- with a second story, built of brick, and an independent porch roof.

So most people in Washington should be particulary interested to hear that the Women's Committee for the National Symphony Orchestra has chosen for its Decorators' Show House one of these early '20s brick houses at 3516 Rittenhouse St. NW, near Chevy Chase Circle. This one not only has a wide front porch, with even a projecting portico and pediment on the front, but also oval windows on the upper floors and other embellishments to show it was a custom-built house.

It also boasts that great amenity of the '20s, a porte-cochere, later to be called a carport. The most sensible of all solutions for car and passenger is simply a roof on supporting pillars, extending from the house, covering a family entrance, here into a study. This being a grand house, there's also a two-story garage in the back. (The upper story to house the chauffeur. But for simpler families, the porte-cochere served well enough.

The National Symphony Decorators' Show House, opening to the public Sunday and continuing through Nov. 2, is always one of the biggest bargains of the year for people looking for almost-free advice ($5 ticket) for their house.

Behind one front door are the best ideas of 31 of Washington's harder working interior-design problem-solvers. Best of all, the problems they've had to solve are no flounces-in-the-sky questions. It's not like setting up pretty rooms in a studio where the world isn't real. The Decorators' Show House is real -- even a real leak on the porch had to be fixed before decorators James P. Pannone, Inc., could go in and paint it shocking pink.

Though Mrs. George C. Boddiger, the chairman, noted, "This year [the decorating] went faster than it ever had before because the house was in better condition than some we've had to contend with. We thought it was a good house because it's a family house, people can see problems solved like those at home."

The house, though not the mansion of last year's show house, has many things to recommend it. Architect John Albert Hunter sited it well -- the garden faces due south, and he wrapped a splendid terrace (decorated by Frank Angel Associates) around the house. The house was built from 1922-25 for Henry N. Brawner, president of Chestnut Farms Dairy. William A. Gruman bought the house in 1937 and his daughter, who married Arthur C. Cox, acquired the house in 1957. The house is currently up for sale for about a half-million dollars from Begg Realtors.

As with most houses, not every room was finished in time for this report. The ones that were complete showed a great deal of Victorian wicker, painted white, green and even pink. The Victorian chaise longue turned up in every other room. The Chinese influence could be seen here and there but was not as all persuasive as you might expect. Colors were much softer, with a few exceptions, than in some years. A soft smokey grey appeared to be the favorite color this year in both paint and fabric. Comfort, rather than drama, seemed to be a major pre-occupation. Antiques, often country or less formal ones, were used here and there, but usually as accent pieces. Most rooms had rugs or painted or stained floors as opposed to wall-to-wall carpeting. Windows were hung, if at all, with less fabric this year, reflecting the high price of fabric, though some designers with a go-for-broke attitude used fabric on the walls. The most unusual wall covering was Penny Poole's log room. The First Floor

The house faces due north, so its deep front porch, though not useful for keeping the sun off, is a cool place to spend a hot Washington summer afternoon. With this in mind, Ed Perleman of Perleman, Inc., has chosen five wicker rocking chairs. "I think they'll look like friendly people sitting, rocking on the porch," he said. He's painted them dark green, as well as the wicker chaise. The cushions are striped in the sort of canvas that reminds me of awnings: green, white, orange and a reddy brown. To keep things brom being too calm and Sunday-sleepy, he's using a hilarious red fantasy chair to sit before the small wicker desk (it'll work for a living as a ticket-sales place). Imported Italian terra-cotta pots will smarten things up. The table is a glass top, 15-inches round, designed for Intrex by John Dickinson.

If you tear yourself away from rocking (Perleman says it's all right: "I hope people with weary feet will take advantage of them"), you'll go into the center front hall. Evermay Interiors and Associates, Inc., (Charles W. Kable and Lynn Vickers Knapp designers) have made a pleasant, subdued entry. The walls are grey, the flowers silk, and the chairs black lacquer -- they're really something. They came from the Evermay Estate in Georgetown, bought by Capt. Peter Belin's father from the palace in Peking. A red Chinese Chippendale mirror is an accent note.

To the left, is the sitting room and bar (where you can go to buy many of the pieces of furniture and accessories). Originally, this room was the dining room of the house and the butler's pantry. The butler's pantry was an essential element in a house of any pretentions of this period. You had to have one whether you had a butler or not. In proper houses, the butler's pantry had a sink where you washed the good china -- the lady of the house often did it herself even if she had a butler because you don't trust just anybody with your Spode. Everyone is pleased as punch to have one because it makes the perfect bar now that things have changed and everybody drinks to console themselves for not having a butler.

Antony Childs Inc. (Childs and Laura Chester designers) has furnished the room as a pleasant sitting room, the 1920s name for a family room, though back then it was much quieter and pleasanter room. A sitting room was the one room in a house that had a fire all winter, so everyone huddled in there. Children did their homework in the room, mother did her mending and father the accounts (or sometimes vice versa). And sometimes people ate Saturday supper here and listened to the radio.

Childs has covered the walls with Laura Ashley's new fabric, which is both a geometric and a flowered print ($8 to $9 a yard). The matching Ashley wallpaper is $13 to $14 a role. The slipcovers of the same fabric were made by Mel Campbell. Danny Alessandro of New York provided the American 18th-century mantel. The mirrored scones are handsome, made by Tiffany. A Laura Ashley single-sized quilt ($150) covers a table. A pine cone cachepot sits on the mantlepice by the 19-th century horse painting, showing a loser on his way home from the race.

Childs wisely kept the handsome glass fronted cabinets in the butler's pantry, and in the manner of the '20s, put a shirred skirt under the counter. The floor is handsomely done with Ashley ceramic tiles, $2 each, $40 for a box of 25.

The kitchen, by Bath & Kitchen Gallery of Rockville has all new fixtures, including a chocolate-colored sink and a Jenn-Aire stove.

The kitchen porch is by Habitat Unlimited of McLean (designed by Susan Smith and Linda Nigard).

Bob Waldron, who has a penchant for informal eating areas, designed the breakfast room with its cheerful southern exposure and view of the garden.

The living room, a generous 29-by-16-foot room, is being designed by Woodward & Lothrop of Chevy Chase with decorators J. Charles Spate, Felicita Amaral and Sally Senner.

Opening from the living room is the 23-by-16-foot dining room, originally the house's back parlor or ballroom. Lascaris Design Group, Inc., has set the table and the scene for quite a dinner party. You'd have to have caviar and Hummingbirds tongues to justify the decor.

In the back corners are wonderful brass palm-tree lights, much like the ones in the kitchen at Brighton Pavilion in England. Frederick Zimmer, a Washington sculptor, made the brass palms using real palms as a base. Dana Westering painted the clouds on the ceiling.

The table ($7,900) is by Erwin Lambeth. The generous sized English wheel-back dining chairs, by Casa Stradivari ($448 and $481), give plenty of room to each person's elbows. The Czechoslovakian crystal chandelier ($2,475) has the silk shades so ubiquitous during that era. Neiman Marcus lent the 12-place settings of Ruby Ulander Wedgewood dinner plates ($86 each); the Baccarat Malmaison, and Baccarat Hircourt Rhine wine. glasses ($105-$150); the circa 1860 English mother-of-pearl fish service ($1,200) and Herends Chinois fishcourse platters ($88.50 each). The desert service is Christofle Spatours. Angelo Benito arranged the silk flowers and shells atop a mirrored plateau.

The Karl Mann upholstered walls and curtains were installed by Horst Bolindorf with hand-made silver tassel tiebacks by Scalamandre -- $6,469 in case you'd like to have your walls and windows so glorified. The handsome four-panel beveled mirrored screen by Regent is $2,945, so walk by it carefully.

The sunroom, complete with shocking pink walls and a bird cage for a stuffed bird, was designed by James P. Pannone of Rockville. The Second Floor

On the second floor, Lord & Taylor of Washington (Daun V. Thomas designer) decorated a sunny study with Indian cotton ($12 a yard, exclusive to Lord & Taylor). The peaceful abstract paintings are Thomas's work as well. The well-scrubbed country 18th-centry desk is from North Carolina or Virginia. Thomas plans to mulch the balcony and have the ivy twine around the furniture.

Holly and Robert Guthridge of Alexandria with Bruce Grissom, Lynn White and Forrest Johnson designed the pretty guest sitting room next door.

The guest bedroom and bath has an armoire with drawers for clothes and a shelf for television; it was designed by Guy Chaduck of California. Rousch & Averill of Gaithersburg were the designers.

Margot Wilson Interiors did the daughter's bedroom with a remarkable canopy made of a lamp shade hung with ribbons. The floor has decoupage flowers cut from wallpaper, then covered with six coats of polyurethane, all by Carol Markell. Beth Wolfe of Centerpiece Flowers made a ceiling light fixture in the bath of real birch branches hung with small Christmas tree lights. The effect at night should be hipnotic.

The second floor hallway is by Boyle Interiors of Fairfax.

The family living room and bath is by Classique Interiors.

The gentleman's bedroom and bath has photographs on the walls, oriental wallpaper and rug, designed by Ellen Cantrell of Clinton, Md.

Hecht's of Washington and Baltimore (Tom Backner, director of design division) designed the lady's bedroom suite with its stack of alligator chests -- some opening up to show mirrors.

The lady's morning room, once a sleeping porch, has imported Victorian wicker, a chaise longue ($550), a desk ($450), and several chairs, including a charming one for a child. The "Eiffel Tower" plant stand ($176) is especially charming. The Third Floor

With the current interest in country crafts and country style, the small third floor log room may be the most popular. The child's playroom by Penne Poole of Washington, with Elizabeth Sprague, has walls covered with slices of logs, chinked between. Peter Larsen, the carpenter, and Bob Oliver, the plasterer, had built log cabins before -- but not on the inside. Poole found them after consulting six telephone directories and more than a dozen mills.

Cherishables, a country antique shop owned by Marilyn Hannigan, lent the decorative objects: a watermelon sculpture and Big Foot Beggar ($4,000) by Miles Carpenter (a 91-year-old Virginian), a fishing boat by Leslie Payne, a rare 18th-century Austrian lambing chair (for the farmer to await the birth, not the sheep), and a Cor-Cor Train collected by Jerry Goldberg, and Marilyn Hannigan's topsy-turvey doll. Who could image trusting a child in a room like that?

Arthur Buchanan arranged the son's room on the third floor with a stair-step carpeted platform, so even a small boy could reach the windows. The single bed is upholstered and covered in a wool-suiting material. But the most interesting thing about the room are the rotating cabinets -- stock maple cabinets set by Buchanan on a swivel base so they can be turned to the wall if you don't want people to know what you're reading.

Buchanan explained that the room had awkward proportions, poorly located windows and a garret ceiling line. Warm beige helps to mask the problem layouts and the platform makes the dormer seem less ungainly.

Design Tile of Tysons Corner (Letitia Marks Hornyak designer) uses an Italian glass mosiac to tile even the old bathtub.

The stairway, landing and third-floor hallway are the work of Interior Concepts of Edgewater, Md. (Arlene Pararella and Richard Medlock designers). Here again you can recline on a chaise if the steps are too much for you.

Other rooms and their designers are: Cedar closet, Jay Crawford Interior Design; Young Man's Bedroom and Dressing Room, by Art Slingluff of Coles Ethan Allen Gallery, Arlington; Getaway, by Heather Suydam Interiors; Hall and Powder Room by Zuko Designs, designers Jean Tinsdale-Green and Phillipa Brisbane; Tea Room, Concepts of Bethesda, Darlene Martin designer.