End of September -- time to fold away the deck chairs, bring in the plants, take down the awning, put up the storm windows, put down the rugs and take a hard look at the inside of the house.

During the summer, it's easy enough to ignore the whole question of the holes in the sofa, to avert your eyes when passing by the peeling paint on the dining room ceiling and to change the subject when family members point out the faded curtains. But the end of September is the time when something must be done.

There is a way to give the appearance of being up and about it, and still give yourself time to postpoine those hard decisions (Mend the sofa? Buy a new slipcover? Throw the sofa away?) $5You can begin by spending the day looking at decoration as practiced by 27 or so of the Washington area's most energetic interior designers in the annual Decorator's Show House. Nothing is so calculated to send you home full of purpose than looking at a really elegant house. All through the show house are simple solutions to design problems that afflict more modest homes.

The house is at Oxon Hill Manor, 6901 Oxon Hill Road, Oxon Hill, Md. (Take the Beltway, Exit 37s).

After the event, Oxon Hill Manor will be restored, with the help of a Maryland Park and Planning Commission grant, and run as a non-profit cultural center with artists' studios and meeting rooms.

Architecturally, the house is a 1920s Georgian gem, designed by Jules Henri de Sibour, who along with George Oakley Totten, designed more embassies in Washington than any other architect. The immense marble-mantled fireplaces, the tall ceilings, the elaborate woodwork, the sweeping staircase, and most of all, the tall French doors combine to make a splendid house even without the attraction of the decoration.

Historically, the house is of great interest, as recognized by its listing on the National Register of Historic Places. Rumor has it that John Hansen, the first president of the United States under the Articles of Confederation, is buried here near the ruins of the 1710 house, destroyed by fire in 1895.

The current house was built in 1928 for Sumner Welles, the undersecretary of state who was President Franklin D. Roosevelt's diplomatic troubleshooter during World War II. Many times Roosevelt came to swim in the pool. According to the story, Winston Churchill visited here and discussed the Normandy invasion on the terrace. Later, it was owned by Fred Maloff, an eccentric antique dealer. At one time the estate was considered by Congress for the vice president's residence.

The house, as sometimes sadly happens to big oil grandiose places when they lose their patron, has been crumbling away. The fine Chinese-painted panels in the dining room are streaked with moisture. Much of the splendid plasterwork in the living room was broken. The handsome carved mantlepiece in the library was vandalized. The floors needed refinishing. All the walls needed paint. Most of all, the house, like most of us needed to have a purpose in life.

Of course, there's still a lot of basic repair work to be done. But you wouldn't know it to look at it now. The Women's Committee and the decorators have painted, wallpapered, scrubbed and polished the old house, and it now has some semblance of its old manners. As usual, the decorators were working right up until preview party time last night, moving furniture, shifting pictures, rolling out rugs and pulling out their hair.

You come into the house through a foyer bigger than some apartments. Drysdale Design Associates of Washington took on this room, with its great fireplace.

The library, designed by Sarah B. Jenkins and C. Larry Horne of W. & J. Sloane Inc., has magnificent wood paneling with pilasters (columns set into the walls). Double French doors go out onto the terrace where Churchill, Roosevelt and Welles once sat, now furnished by Magenta Yglesias of Designare Ltd. (S.D. Hecht). Ther double doors lead to the living and dining rooms. More doors go to the foyer.

The Jenkins/Horne design answers a common problem: What do you do if the room is large but the wall area is small? Their answer is to float two handome grey/beige tweed upholstered modular seating units in the middle, in front of the fireplace, but aiming more at the terrace view through the French doors. Light built into the base of the seating makes it seem to float. On the wall is a magnificent Chinese kimono.

A ceiling photomural of outer space sets the theme of Outer/Inner space, and a mythical occupant who is a space professor. The bookcases have mirrored backs to lighten the dark paneling. The floor is carpeted wall to wall in a light color to match the modular seating. The ceiling, with its stylized Art Deco panels, is painted white.

The drawing room stretches across one end of the house, with French doors on three sides. A highly figured Georgian frieze goes around the top of the walls. A chair rail set low makes the walls seem taller. Over the fireplace is a painting of Edward Nightengale, a fancily dressed gentelman of 1727, painted by John Smibert. The Lord and Taylor decorators, Gail Jackson of Falls Church and Daun Thomas of Chevy Chase, have painted the walls ashes of roses. The exotic furnishings include a 6th-century Buddha ($4,000), a rain drum from Thailand, a pair of 1810 Biedemeyer cabinets, and a $10,000 Kerman rug from Iran.

The dining room, a magnificent room that could seat possibly 50, if they were all diplomatic, was decorated by J. Charles Spate of Woodward & Lothrop, using the exquisite colors of the Chinese panels which are set into the walls.

An intimate reception room, a cabinet, as it was once called, no doubt was the sort of place where on a long winter night, Welles and Roosevelt could settle the affairs of the world. It would have been a cozy room, despite the talk of wars and rumors of wars. Paintings are mounted directly on the wall all over the room, framed with molding. The paintings were by an old friend of Welles, R. W. Ives Gammell. They are rather pre-Raphaelite/Art Deco versions of the Daphne and Chloe legend.

Edward Warren Richardson, of Washington has made a bow to the age of the house with sparkling glass ornaments. The two fan-shaped pieces are Waterford. The glass head is signed H. Hoffman, 1925. The glass is echoed in the lucite curtain rod. The marble mantle surrounds a peacock-mirrored fan designed by Richardson for the spot. In two inserts in the wall, Richardson has hung a modern chair mail will shimmers and sparkles to set off two Chinese vases. A banquette against the wall and a pair of Louis XV chairs offer seating.

The cloakroom -- fancy houses have cloakrooms instead of hall closets -- has been done up with an oriental motif by Jacqueline Gaskill of Alexandria, Va. On one wall she used an 1800 bamboo painted screen. A Roman shade with an Indian pattern covers the window. Brighton Pavilion bamboo furniture is covered with fabric named "Walpole." Indian silkscreen blocks and framed Chinese embroidered sleeves add interest to another wall. The bath is covered with shells of all sizes.

The nearby sewing room is decorated by Harriet Hertz and Linda Kramer of Harlin Enterprises of Rockville. They covered the wall with a neutral colored fabric and painted the cabinets a deeper color.

The staircase hall, with its handsome marble floor, was decorated by Margot Wilson of Washington. The staricase itself needs no ornament. It sweeps up in one sinuous curve. The wrought iron railing says an elaborate S all the way up (did de Sibour put one over on Welles?) A table with a long skirt and an elaborate Japanese-style flower arrangement sits between two chairs. Ferns on the window and at the foot of the stairs add greenery. The elaborate panels and dentil moldings have been picked out with alighter color for emphasis.

The upper hall is the work of Parvizian of Chevy Chase, with Andy Wallitzer as decorator. Barbara Jean Cambell did the nook.

The guest bedroom, decorated by S. Thomas Getz and Charles W. Kable of Classique Interiors of Potomac, has a palaquin canopy -- the kind some people call an empress canopy, a semi-circle at the end of the bed. The headboard copies the palaquin shape with its own semi-circle of shirred fabric. The bedspread's pattern matches the canopy fabric.

Lannie W. Neal Jr. designed the young person's bedroom, though it would seem too good for anyone very young.A handsome brass cabinet, with place in top for books or papers or knitting, a side lantern and a built in ash tray, started life in the Orient as a cart for sidewalk noodle sales. The brass is repeated in the handsome etageres which hold porcelain deer and even an ostrich egg.

The floors of the television room are finished with a limed white pickled finish. Frank Babb Randolph said he first had the floors stripped, then painted them with a white waxy paint, rubbed in with a turkish towel. Afterward he wiped most of the paint off and added two coats of clear polyurethene bowling alley finish. Most of the furniture in the room including a white pine chest, is also light-colored. The fireplace is marble, as are so many in the house. Randolph has contrasted these traditional touches with a modern painting by Willen de Looper and sketches by Robert Tollast.

The sitting room is by Lila Weisberg, Florence Forster and Diane Shields of Design Associates, Potomac. The immense marble bathroom is decorated by Arlene Golub of Rockville. The gentelman's bedroom is by Roush & Averill of Gaithersburg, the lady's bedroom by Holly M. Guthridge of Wellington House, the gentleman's bath by Arthur Buchanan, AB Design of Washington, the dressing room closet by Joan Nevins Interiors of Rockville with Beth Nevins. Another bath is by Fran Schorr and Ann Lane of Interiors a la Carte, Chevy Chase, and still another by Ellen Carroll of McLean. The upper foyer was done by Sally Hansen, Margaret Ham and Ben Summerford. The tea room, where food will be served, is the work of the Porter Group, Lizzee Porter and Ingrid Fraley of Georgetown. The flower room is by Jean Tisdale-Green of Zu-Ko Designs and the pantry by Ellen Cantrell of Clinton, Md.