JOAN TOBIN'S office and her house both look ever so slightly like one of those rooms sometimes portrayed in a Sotheby Parke Bernet catalogue. That's all to the good because Joan Tobin is the director of the new Sotheby office in Washington.

Her small reception room in the newly done up building at 2903 M St. NW is the sort of setting suitable for the discussion of the disposal of vast estates: elegant soft melon sofa, warm Pompeii red walls, framed antique needlework, matching gilded mirrors over the Italian carved wooden chests, Chinese woodblock prints, shell-encrusted desk accessories, china vases (from the Christ Child Opportunity Shop.)

But the room is small enough and Joan Tobin so earnest and anxious to please, that you might not be afraid to bring up the subject of Aunt Jean's dubious garnets.

The room, isn't the result of successful bids at Sotheby's. "I think I used to be frightened of auctions. Afraid I'd scratch my nose at the wrong time and pay $100,000 for an armoire," Joan Tobin admitted. "That's one of the things I hope to do here in Washington, is help take the mystery out of buying and selling at auction."

Joan Tobin herself is so cautious that you'd think her life was a Swiss bank account number. Her credentials as a businesswoman are formidable, including the best of all: family money, and a long string of successes in business enterprises.

But she'd rather talk in generalities than specifics. Not only won't she tell the funny family story, but she stops her husband lawyer Maurice Tobin, from telling them. Yet, she goes out of her way to be nice, friendly, comfortable and helpful. "I haven't gotten my act together on publicity," she said. "I wish I were more confortable talking about myself. I've always kept my business and private life separate."

At 36, she looks much younger, with her full dark hair, often pulled back from her face with her glasses, and an English complexion over good bones.

She seems much more comfortable talking about business subjects than her house. She is one of those always accustomed to wealth who believes with Bunnie (Mrs. Paul) Mellon that "nothing should be noticed." She finds it hard to talk about such things as where on object came from or how much it cost.

The decoration of their house, led to one of her other enterprises. "I met Mark Hampton at a party in New York," she said. "And we hit it off immediately. I asked him to design our house. And after we'd worked on it for a year -- it's taken almost six years in all -- I became chairman of his board." Hampton is currently one of the best-known New York interior designers. He also designed her new Sotheby's office.

She credits Hampton with arousing her interest and appreciation of objects.

"He's so good at helping you learn how you feel about things." Now, as she and her husband fly about the country making money, they stop to spend a bit here and there in antique shops along the way.

"Maurice is good at the dramatic touches in the house," she said. "I think I have more talent at unity, pulling it all together."

We came into the house past the sleeping lions and into the two-story-high front hall.

Many of the objects in the office came from the 1920s stone house in the embassy section, but they could hardly have been missed.

"We found the tall clock all dirty and in disrepair," she said, "but I loved the painting on the bottom panel. We've tried to get it to strike with all the others in the house, but it'll only work for a while. It takes very delicate balancing, just the right matchbox under the case."

A splendid antique cabinet from her family not only has glass doors for display (Chinese export porcelain here) but also wonderful small lettered drawers with bigger ones below.

There are so many bits of fantasy, humor and grace notes in the house that though its many objects make you want to stop and look at each one, the effect is not forbidding or frightening but friendly. Joan Tobin didn't intend the house to look like, as the catalogues say, "a rare and important collection."

"You're supposed to walk in and want to flop down," she said, as she did that very thing in her elegant drawing room. "Please sit down, those chairs look too stiff if no one is sitting in them," she told us.

"It's hard to imagine anyone wanting to "flop" in that room despite the number of soft seats, including the custom-made chaise that seats 12 for a buffet super or two for lounging. Too many temptations hang on the wall or stand on the table tops like miniature treasure troves. No one would want to sit down before he'd wandered around, stared at the paintings (two Early American finds from the Christ Child Opportunity Shop, among others) and the framed antique needle-work, perused the family photographs in silver frames and inspected the family mania: shell craft.

The Tobin house has as many shells as a tropical beach: antique pictures, tissue boxes, cigarette boxes, desk boxes, all encrusted with shells. Maurice Tobin buys them for Joan, and when he can't find enough, he makes them for her.

Small Buddhas are grouped on the desk in the sitting room. And a large one benignly surveys the living room from a console table. The Tobins were among the early vistors to China and were much impresed. They often eat at Germaine's and curse the fact that they didn't venture capital on that restaurant.

In the more intimate Library or sitting room, at one end of the vast drawing room, a crystal chandelier in the shape of a ship sails through the air and a real nightingale sings in hop of supper.

On every flat surface are pretty china vases with masses of flowers.

In the dining room, the handsome oval table is a family piece. Fifteen chairs are scattered around the room, but there's so much space, you have to count the chairs to believe there are that many. A smaller, delicate table with just two chairs stands in the bay, looking out to the garden. On either side of the window are citrus trees with, surprise!, herbs tucked in around the base.

From the dining room bay of French doors, you have such a good view of the terrace that we felt compelled to go out to it. First, though, Joan Tobin had to find the butler and his keys. Her passion for caution includes security. Th big black Labrador retriever is only the most obvious of the elaborate protection against The Burglar.

The terrace may well by the best part of the house. It is very formal, yet completely usable with a minimum of care (by mansion garden standards). Maurice Tobin worked with architect Bill Pickens on its design. The flagstone terrace is in tiers.

Tobin is obviously the gardner of the family. His wife said, "He told me the names of everything to tell you. But I've forgotten them all."

Pear trees are espaliered against the house.Tucked around the base of the pear trees is Swiss chard. "You can pick it all summer," Maurice Tobin said.

Other pots hold butterfly bushes. "We're away so much," said Tobin, "that we can't have flowers that just bloom at 11 p.m. on May 9. We like things we can enjoy all summer."

The upper terrace has the surprise. Only when you walk all the way over to the wall do you see a magnificent swimming pool that masquerades as a reflecting pond. It's only seven feet wide by 40 feet long, running horizontially across the back of the garden. But it's big enough for laps in the hot Washington summer.

A topiary boxwood stands in a box to the side.Maurice Tobin says that a former gardener came up to them one afternoon and said, "Well, I finally got that long branch whacked off, the one sticking out of the head of your horse." So much for unicorns.

The boxwood was a fortunate buy. Maurice Tobin said that he bought them from the grounds of an insurance company at a good price. "I think they thought they were spirea or something," he said.

When the Tobins give a big party, as they often do, frequently on behalf of the theater (he is head of the National Theater), they spread a tent over the top terrace, with the dinning room looking like a stage with candles instead of footlights.

Alma Viator, who works for Tobin at the National Theater and Joan Tobin at Sotheby's, said, "Joan really does it right. "The tent has to have just the right number of poles. And the tablecloths have to reach the floor. The tablecloths, of course, are made by Hampton, just for the Tobins. She's a real stickler for detail."

Inside the house, the cook was trying out an egg in aspic dish to see if it would hold up long enough to be served for a big dinner to celebrate the coming of Sotheby's. The kitchen, with its multiple pantries and sink, is equipped to serve more people than you'd like to have to dinner. Off it is a small breakfast room, added to a design by Pickens, below the dressing room extension on the second floor.

Upstairs are more marvels. The study/sitting room has a partners desk with separate but unequal phones. His side has the multi-line console. The fireplace has a bronze face. "It was in the barn at my family's place in Cincinnati," Joan Tobin said. A Miro, flanked by drawings, hangs over the sofa.

The fireplace in the bedroom, with its elaborately carved wood relief, was added when the Tobins extended the suite to make a dressing room and bath for Joan Tobin, to a design by Pickens. Maurice Tobin has a more spartan arrangement at the other end of the bedroom, but he gets the wonderful big Dutch armoire they bought on their honeymoon in 1972. In between is a flower curtain bedraped canopy bed.

Tobin grows morning glory vines out of two pots set in the window. They're already blooming. Two other guest rooms complete this floor.

On the third floor are a sitting room and bedroom suite and two other bedroom suites. A skylight over the hall allows it to be a mini solarium -- Maurice Tobin's idea. "I opposed it; I was so uptight about storage space," Joan Tobin said. Two more birds sing up here.

The birds and the Tobins have good reason to sing. Theirs is the sort of life everyone dreams of living in Washington, but seldom manages. But Joan Tobin, out of her new Sotheby's office, would like to provide similar settings for others.