OCTOBER IS the month when you can't ignore the hole in the living room couch any more. During the summer you can drop a large magazine over the hole, or artfully arrange a pillow on the spot, with the full confidence that people will only pass by on their way to the great outdoors.

But any day now, Indian summer will leave the reservation, and we'll have to go back to living indoors. That's the time when we too often realize that "the heap of living to make a house a home" has also made the house a heap.

It is not necessary to go it alone. Help is here.

Three interior design chiefs - Ben Flowers at Bloomingdale's, Tyson's Corner; Claus Mahnken at the downtown Woodward & Lothrop's; and George Barber at the downtown Hecht's - have folded their patio umbrellas and moved inside to their model rooms to show you how life can be beautiful. . . if you use their department stores charge account. Model rooms are a good way to judge whether you can live with that designer and that store.

Sometimes you'll just see what you want in a model room, as did the man who came to Woodie's and ordered up one whole model room, from rug to chandelier and all in between. Sometimes you'll hate a model room, as did the man who chased Claus Mahnken all over the store with a furled umbrella because Mahnken dared use a modern fabric on a Williamsburg reproduction sofa.

Hechinger's, "the world's most unusual lumber yard." as they call it, is getting more and more unusual - it's beginning to look like a department store. (Where's the 2x4s? "Over there, behind the pillow covers.") Now Hechinger's has added anovel twist to the department store in-house interior designer - design by mail. It is the bright idea of two Washington interior designers who are also writers on interior design: Emily Malino ( who appears regularly in Living In Style) and Gloria Veissberg (who has written for The Star's Home/Life.)

Their "Decorative by Mail Kit," which costs $3.95 or $4.95 after Oct. 10, includes "A Room Analyser Questionnaire," an exhaustive nine-page examination of the client and the house. The client also is asked to draw a floor plan of the rooms to be decorated (detailed instructions on how to measure and draw) and to paste up sheets with samples of coverings to be retained. The cost of $35 for each of the first three rooms and $25 for the fourth.

Back in the mail comes the custom room plan showing the furniture arrangement, lightning, plants and wall decorating and a "presentation board" with a color scheme, samples of wall coverings, furniture furnishes, upholstery fabrics, floor coverings and window materials. also offered are Decorators Touch discounts on items fron Hechinger stores specified by the plan - 10 per cent off on carpeting, painting, wall-paper, etc., if you buy enough.

There are, of course, numerous decorating books coming out every week. Rather different is "The Instant Decoratot," by Frances Joslin Gold ($27.50, Clarkson N. Potter Inc., publisher, availables in bookstores). It is actually a workbook with sketches of 16 see-through rooms so designed that swatches of fabric and paint can be slipped behind to see if all will go well together. It's almost decorating by the number.

The largest collection of model rooms by Washington interior designers comes every year in Decoratord Showhouse, 650 Chain Bridge Road, McLelean, a project of the Women's Committee of the National Symphony Orchestra. The benefit will show the work of 30 designers. Every thing will be for sale. The public, for a $5 fee, will be welcome Saturday through Oct. 23, from 10 a.m. to 3:30 p.m. Sundays. Closed Oct. 3. imformation at the National Symphony Office, 785-8100.

When using the services of any of these professional helpers, look carefully, listen intently and then decide how the advice fits in your life and - most important - your pocketbook. Designers should be a source, not dictators. If you don't want to do what they tell you to do, don't. If it's too expensive, say so. It's your money and your house. They're working for you.

Full disclosure from the beginning is best. Explain how much you want to spend. Tell the professional honestly how you live or how you want to live. (If you are happy with the hole in the sofa, you wouldn't need a new sofa. All new furniture should be intended to make you live better. So the critical part of your clean breast to the designer might be to explain what's wrong and how you cope to correct the problem.)

The greatest benefits to be derived from working with a designer may be the education of your own taste. In discussing with the designer how something is to be done, you may worked out the solution yourself. It's rather like going to the psychiatrist to talk out your problem and finding you're being led to give the answer yourself. Malino says she's perfectly sure some people will buy her "Decorate by Mail Kit," do all the homework and decide exactly what they want to do on their own, without mailing it in.

Professional designers, on the other hand, should not be discounted. They know a great many things many of us don't - from how to create a certain effects to where to go for that rare and exotic "touch" that makes the room snap together. (Malino/Weissberg's kit guarantees one whoop-te-do idea per room.) The most difficult part of decorating without help is finding where to buy the balck indoor-outdoor carpet, the clean lucite blind or the egg-shaped lamp you can't live without. (All the foregoing, is unlikely as they sound, are shown in the model rooms of Woodie's main store.)

Here are few more ideas from the new crop of model rooms in the Washington area:

At Woodward & Lothrop's downtown store:

In a room designed to show how to raise the sights of a one-room basement apartment, Claus Mahnken used a modular sofa by Flair. In some cases, the sofa units are put together front-to-front to make a daybed. Unholstered platforms serve as tables. Neatest of all is a huge square raft, which could be a snadbox, a trampoline, a dry swimming pool or a passion pit. around the outside is ribbed brass. Inside is a mattress covered with suede. It might be considered the ultimate in the bed-in-the-living room school of efficiancy apartments. the custom blinds here are lucite see-throughs with glittering tapes to match the Chinese screen. The walls, like the floor, are covered with a lightly woven charcoal-gray, indoor-outdoor carpet.

Many rooms this year have dark walls, wall-to-wall carpet, heavy window drapery - all with the idea of keeping out the drafts, holding the warmth in, using color and texture to give the illusion of warmth. The most opulent may well be the Mahnken-designed bedroom using the Baker furniture company's Charleston rice plantation pieces. The elaborate window drapery by Ernest Fernanez even has a rosette on the valance. The ornately carved four-poster bed with rice rows recalls Benjamin Latrobe's corn capitals in the Capitol.

At Hetch's downtown store:

A Caribbean room designed by George Barber with the help from Tom Backner uses wood posts to delineate one side of the room. More wood lengths are set in angles used to divide and soften the stucco walls. Shades pulled up on the lower section of the window, blind folds on the upper window.

In a modern dining room, Ello's chrome and glass dining table, ($660) and black floors make you feel as though you are inside a looking glass. Ello's mirrored chest add to the reflections. The handsome high-backed banquettes are designed by Milo Baughman for Thayer Coggin.

In an Oriental room, ordinary, inexpensive, textured vinyl wallcovering was lacquered black by Barber and then glistened with a polyurethene clear coat. The effect is brilliant. An Oriental screen is used in a bay in place of curtains.Poles of bamboo centered with strips of brass make an unusual frame for a door. In this room, the cocktail table is a big square table with a curved foot. Hetch's calls it an elephant table, Bloomingdale's, which likes it too, says it's a chow table.

At Bloomingdale's, Tyson's Corner:

Ben Flowers, with help from Susan Pennington and Danya Pelzman, has used about $1,200 of ribbed mirror to brighten a room dedicated to TV personality Mike Wallace. The room is complete with Wallace's two Emmy awards, a gold typewriter TV Critics award and a photograph of Wallace being hauled out of a Democratic convention by the police. (And isn't that that choicest bit of decorating. Perhaps in your own home you could substitute a picture of yourself discovering the Denver boot on your car or yourself being audited by the income tax people.)

Large oriental fans are turned into $375 sconces by George Kovacs Inc. in the room dedicated to singer Roberta Flack. Handsome glass lamps ($195) are hung in threes to make a three-times better chandeleistyle chairs come unfinished ($280). Kelim rugs, traditionally small, are here used one on top of each other for a pattern-on-pattern effect.

Super-sized octagonal tiles from Sally Coler's Tile Gallery make a fine basis for a room designed by Flowers et al for authors Alex Haley. The photographs are Haley's family and the model of a mosque is by his architect brother, Julius. Moral: Show your roots.

Perhaps the single most unusual and handsome piece of furniture at Bloomie's (not to mention the most expensive) is the Founder's stainless steel and glass buffet ($1,895). The walls of this room, honoring tennis star Arthur Ashe, has $67-a-yard Jack Lenor Larsen fabric. The fabric, however, isn't as expensive as you might think, because it is railroaded. That means the pattern runs the length, not the width, of the bolt, and the fabric is 10 feet wide. This makes it possible to have seamless wall covering.

The room by Flowers named for Dick Gregory has two storage walls with a bridge of light making a head board ($275 per unit.)

In all three stores, baskets, some of then big enough for Ali Baba, are used to hold plants, people (the baskets turned upside down as ottomans), papers, and who knows what else.

If all this seems too much, you could just pitch a tent on a Florida Key and avoid the whole question of the hole in the sofa.