THE GRANDE DAME of a mansion sits expansively at 22nd Street and Decature Place, at the the foot of theSpanish steps, spreading her terraces and gardens like a full skirt. The house rises majestically for four floors.

Her two wings project out to form an impressive arrival court of herringbone yellow brick with an elaborate border, imported from France. The tall iron gates -- 15 or 20 feet high -- clank closed at night to keep out the hoi polloi. All the windows have working shutters made to be closed and opened depending on the position of the sun. Stone panels of carefully carved foliage are set into the dark redbrick walls of the house. A high and handsome foundation and garden wall is relieved by a stone balustrade and urns. h

On the piano nobile, the principal floor, raised well up above the dirt and the distractions of the street, the great arched French doors parade around the house. Balconies over the entrance, a terrace on the southwest end, and the brick courtyard invite the grand owners to fling open those great doors to the mild October weather.

After successfully traversing the iron gates (imagining myself in sables arriving in a Rolls-Royce chauffeured limousine), we walked inside, with Stellita (Mrs. George) Renchard leading the way.

"My house, as you know, at the top of the Spanish steps, overlooks this one," she said. "So i've always watched out for it, and worried about the garden. The house had been for sale for five years, after the Louise Home moved out. Well, i heard the property was to be sold to a group who were going to raze the house and put in 30 townhouse condominiums. I decided right away i had to save it. There wasn't anybody but me who would do it. Preservation begins with one's self. So in 48 hours i raised enough money to put the down payment on the $550,000 price. The other prospective buyers were amazed when i walked in with my check."

That was last February. Renchard soon found out that though it was romantic, and certainly public spirited, to make such a grand gesture of preservation, there were practical considerations. All right-thinking people were ready to pin a medal on her, but no one came forward to pay the heating bills.

"The heating oil trucks were arriving every five days to fill the two 500 gallon tanks," said Renchard. "I certainly couldn't afford that. So i drained the tanks -- and found out how precarious the plumbing is."

At this point, Mrs. Renchard isn't sure what should be done with the house. She has saved a good many architectural artifacts before, including a plaque from a Dupont Circle house being demolished and a choice bit of stone carving from K and 15th Street, but never before anything quite as large as this, the Codman house.

She found the original drawings for the Codman mansion (on fine and fragile linen) specifing every detail, even the mortar joints, in the Avery Library at Columbia University. She knows she doesn't intend to sell it, and she does intend to preserve it. "I think perhaps one of those big think tanks. Or a museum of Latin American culture, that would be grand."

Mrs. Rechard knows a great deal about other cultures. Her husband has been ambassador to 10 African countries, and they have served at 18 different posts. Mrs. Renchard is on the board of the Smithsonian's African Museum, among other things.

At the moment, she's paying the house's taxes at the higher commercial rate, not to mention the 13 1/2 percent mortgage. But things are looking up. In June, the house was declared a Category II Historical Landmark, which gives it some protection against wanton destruction.

We went inside the double doors, past the fancy stone entry way. The small foyer has an arched niche for a marble statue. Mrs. Renchard has a marble statue, of course, but she hadn't decided whether to put it there, for Dupont Circle Citizens Association House Tour. Tickets for the whole tour are being sold in the foyer.

The house was the Louise Home from 1947-76. W.W. Corcoran, who also gave the city the gallery, established the home at Massachusetts Avenue and 15th Street. The home moved to Decatur Place in 1947. The home was named after his 16 year-old bride and his daughter, both of whom died young.

Corcoran was a Southern sympathizer who found it expedient to spend the Civil War years in Paris. He was much concerned about the plight of the gentlewomen whose Confederate officer husbands had been killed in the War. So he established the home "for the comfortable maintenance and support (not including the furnishing of wearing apparel) of . . . destitute but refined and educated gentlewomen . . . from that class of individuals who have known brighter days and fairer prospects." (In 1976, the Louise Home was combined with the Lisner Home and moved out to Western Avenue.) The Two Codmans

Martha Catherine Codman built her Decatur Place mansion in 1906-07. The architect was her cousin, Ogden Codman. He took the design, it is said, from the Chaeau de Voisins Louveciennes, Seine et Oise .

Martha Codman was well-equipped to be a client of Codman's, having inherited a considerable amount of New England money from her parents, whose wealth came from the Russian and China clipper trade. A doctoral dissertation by Janet Davis, then a student at the University of Virginia, gives much fascinating information on the house, its owners and architect.

Martha Codman had grown up in Newport and Boston. She felt no need to marry as she had a place in society and the money to maintain it. She also may have been dubious as to whether she was being courted for her money or herself. She was 50 when she built the house.

Ogden Codman was the society decorator of his day, and he occasionally acted as architect to design the exterior as well, all in the most refined taste. His first important commission was to design a house in 1883 for Alfred M. Coats in Newport, and he went on to decorate many Newport summer "cottages", as the mansions are called.

He met author Edith Wharton when he decorated her Newport house, "Land's End." They collaborated on a book, "The Decoration of Houses," recently republished. Her novel, "Ethan Frome," and most of her short stories came later. The decorating book advocates a classical design approach, sweeping away the Victorian clutter that then prevailed, and emphasizing the need for architects and interior designers.

Codman's idea of simplicity took a great deal of marble, stone, exquisite carving and terrazzo. He couldn't imagine a house with fewer than 20 rooms. Of course, Codman's experience was in houses of a considerable size. He decorated 10 bedrooms of the Breakers for Cornelius Vanderbilt as well as John D. Rockefeller's Kyhuit estate in Tarrytown, and the famous Queen's bedroom for Mrs. Frederick William Vanderbilt.

So it was natural enough that his cousin should choose him when she decided to make a place in Washington society at the turn of the century.

She chose two lots cut from the S.S. Kall estate in Kalorama. Davis notes that the Kall estate was one of the last large estates in town. The steep lot is not that large -- 71-by-156-feet -- and the house occupies most offit. One observer notes that estates continue to get smaller -- if Renchard had not intervened, the 22,000-square-foot lot would have been further subdivided into 30. The house was begun in 1906 and finished in 1907.

The house apparently brought Martha Codman all she wanted. She had musicales in the fine front room during the winter season. And she spent summers in Newport, in another house designed for her by Ogden Codman.

In the mid-'20s at a musicale, she met a young Russian tenor, Maxim Karolik. He was a refugee who made his living by singing at society parties. kThey were married in 1928, when she was 72 and he was in his 30s. By all reports, they lived happily. They assembled a great collection of early 19th centurty arts and crafts. They left the collection to the Boston Museum of Fine Arts, which built a wing for it. Martha Codman Karolik died at 92.

She sold the Washington house in 1938 to the former governor general of the Philippines, Dwight Filley Davis.Davis was the donor of the Davis Cup for tennis. He added a one-story brick extension in 1938 on the north (buying another bit of property for the purpose) probably to bring the kitchen up from the basement and to add a servants' wing. His wife installed Chinese wallpaper in the music room. She took it with her when she sold the house after her husband's death. Symmetry

Mrs. Renchard was told by one authority that the house is the second most beautifully proportioned in the city, and not many would argue against it. (The first being the British Embassey, the authority said.)

Codman pulled off several tricks to attain the desired symmetry. On the southwest corner, he wrapped an elshaped terrace around the house, protected from the street by a high wall that begins as a retaining wall. At the second-story level, the wall is a high screen topped with a stone balustrade, ornamented by corner urns. On the opposite side, the southeast corner an equally tall wall encloses only air. A paved yard (for drying?) leads to an open air tunnel or passageway underneath the house to the service rooms.

Stellita Renchard led us in through the marble floored entry (with all the squares of marble set at a diagonal, a Codman signature), past the horizontal recesses in the walls, the plaster pediments, and the mirror framed with plaster ornament, to the first floor office, the only fancy room on the floor, with its green and white carved marble mantel.

"This would have been where Martha Codman would have gone over the accounts with the housekeeper, or given instructions to her butler, or received tradespeople," she said.

The staircase, a gracious spiral with a neat iron balustrade, curves up all four floors to a domed skylight, encircled with plaster ornament.

The elaborate plasterwork continues at the second story landing with plaques of classical figures and elaborate cornices. Renchard points out that the brass lanterns on each floor are scaled to match the diminishing ceiling heights. Today, the rooms are almost bare. Although the rooms are vast, they don't look as empty and unfurnished as you would think. The architecture -- the plasterwork, the tall doors, the handsome mirrors -- seem to furnish the rooms.

All of the major rooms are on the southwest corner, and all lead through French windows out to the terrace. The music room, with another marble fireplace in the classical taste is on the street side. The large drawing room across the end of the house is centered by the tinkling chandelier, which has never been converted to electricity.

The dining room, across the back, has even more elaborate plasterwork than the other rooms. A door on the west leads into the conservatory, facing south. At the other end of the room, it leads into the pantry, one of the most interesting rooms, then a second pantry (for the butler's butler?), and then into the kitchen, added later. The walls here and the laundry (on the ground floor) are glazed brick.

In the center front, is a library paneled with a rare wood, prickly mahogany. The library itself seems rather small for a house this size. Renchard thinks it was perhaps made smaller by the Louise Home, because an ante room has the same diamond parquet floor. In any case, the other rooms on this side of the house, though they have the same French doors, are rather cramped, compared to the grandiose west rooms.

Upstairs on two floors are some 37 small bedrooms and uncounted baths, made from the original, more commodious, rooms. Designed it is said, so every room would be the same size to keep the peace amongst the gentlewomen. In Martha Codman's day, these were expansive bedrooms, many with marble fireplaces. Renchard noted that the firm of Faulkner, Fryer and Vanderpool -- which designed the renovations -- carefully matched all the plaster details, especially the cornice work.

On the ground floor, in the back, are the utility rooms, worth the price of admission themselves. When you see the furnace room, it's easy to understand why Stellita Renchard had it closed down. The furnace looks very hungry indeed. It has its own two story high room -- must be 20 feet.

The Codman house is one of the most majestic houses in Washington's most magnificent area. The house, and the city, are very fortunate that Stelita Renchard came to its rescue.