NOT ALL the stories about the new $12-million Japanese Embassy residence are true. The staff does not use Toyotas to go from one end of the great hall to the other. Nor is the salon a test gallery for 1,000-millimeter Nikon zoom lenses.

The monumental modern fortress-like building at 4000 Nebraska Ave. NW, the new house of the ambassador, replaces the old Massachusetts Avenue house, which the Japanese hope will be rezoned as a chancery, the embassy office building.

In fact, at 81 feet in length, the salon of the new residence is not - no matter what you might have heard - as long as the Kennedy Center foyer's 630 feet. (It is, at 33 feet, only 7 feet under the foyer's width.)

The salon is longer than the White House ballroom; the East Reception Room (70 feet); the State Department's Adams Drawing Room (72 feet) and the British Embassy ballroom (49 feet). Nobby, the dachshund belonging to Ambassador and Mrs. Fumihiko Togo, finds it just the right length in which to chase his yellow ball.

The salon's ceiling is 20.6 feet high. The tall sliding-glass doors (protected with sliding shoji screens instead of curtains, for both privacy and insulation) overlook the tennis court, the swimming pool and the rear half of the embassy ground's 7.5 acres.

The salon is only slightly smaller than the great hall (26.8 feet high, 81 feet long, 27 feet wide.) Through its east floor-to-ceiling wall of windows you see a Japanese post-card view: teahouse (prefabricated in Japan), pond (man-made) but which real carp for whose benefit the water fountain was left running during last week's snow) and an artificial waterfall with wet water and real rocks. The view from the tall windows is only of this imaginary Japanese world because the embassy is sited on a hillside.

At the other end of the hall is another vista, a smaller walled garden, with, of course, a stone Japanese lantern. Now there are evergreens frosted with snow and bare-branched maples, but this spring, all the bulbs should blossom with color. A dramatic staircase to the embassy's 20-odd guestrooms rises out of the great hall.

Glittering light sculptures - the work of Tada Minami - shine and shimmer in long icicles from the ceilings in both the hall and the salon, as well as in a smaller salon and the dining room. The art work, according to counselor Taizo Watanabe, all was especially commissioned for the building, and painted by the artists for only a token fee. The dining-room screen, possibly the most spectacular of the paintings, is the work of Matayo Kayama. Toko Shinoda painted the picture set into a niche, Japanese-style, in the small salon.

The grand salon - too big and too empty to be called a drawing room and of course many times too grand to be a living room - serves the embassy as a ballroom. The other night, at the "New Ball in Town" benefit for Cystic Fibrosis, the enormous rug was rolled up and the beige upholstered chairs (all the same, like an airport lounge) and the alabaster cigarette tables banded in brass were removed for dancing.

The prime use of the embassy building is to serve as a showcase of Japanese commerce and culture. It is not really a house, but a series of reception rooms, a small guest hotel, and, almost incidently, an apartment for the ambassador's family.

In September, when the paint was hardly dry, and the grand salon not yet finished, the great hall was used as the site for the after-theater party following a Kabuki play, with an ice dragon protecting the chocolat mousse on a buffet table. In November, 245 people came to a black-tie dinner to see models of couture by Japanese designer Hanae Mori sweeping down the dramatic staircase.

Luncheons are held often in the tempura room, a more intimate space of dark beams and handsome woodwork. The tempura room, dedicated to the service of that Japanese quick-fried fish and vegetable delicacy, was the request of Ise Togo, the ambassador's wife. The room had originally been planned as a men's bar and game room, but Mrs. Togo, an elegant and worldly diplomat, knows that the style of entertaining today in Washington is ethnic. The chief's aisle is sunk below a long bar. The room will accomodate enough tempura fanatics at small tables to drive a bevy of chefs to sushi.

The teahouse is reached through a glass-enclosed walkway. Half-size shoji screens at eye level hide the garden so you won't be distracted from the contemplation necessary for the tea ceremony. But Mrs. Togo says that people tend to bend down and peep anyway.

For a while, the embassy will have to use the old teahouse at the Massachusetts Avenue building because the new one has yet to be blessed, so to speak, by a teamaster. The latest teahouse seems very pleasant, even so, with tatami mats and wooden benches with straw pillows and a fine view of the waterfall, though the platform for the traditional flower arrangement is, at the moment, empty. The teahouse has its own tiny fountain with pretty black rocks set into one corner of the floor. Regrettably, said Mrs. Togo, at one stage it flooded the teahouse.

In Washington, embassies are our modern-day palaces, marble manifestations of the way the nations of the world wish to present themselves to other countries. Embassies here, especially those of rich and prosperous nations, are very generous about lending their facilities for American charitable events: dances, cocktail parties, fashion shows, theatrical performances, rarely for less than 40, more often 500, sometimes a thousand.

The new banquet room (48 by 27 feet) will seat 40 people who haven't been introduced, and a great many more who are friends. For more intimate dining there is the family dining room seating 12 without putting any more leaves in the table. (Nobby's elegant round red stuffed bed is established here with the hope some dieter will not finish his ice cream.)

Across a spacious serving foyer from the dining rooms is the butler's or serving pantry - the size of a good-sized restaurant kitchen. The big kitchen is on the lower floor, along with offices for the household management, servants' rooms and garages.

More intimate entertaining is possible in the small salon, the after-dinner room, the music room or the upstairs lounge.

At the other end of the building, in the private wing, are a sitting room with a fireplace, the ambassador's study, a small dining room and kitchenette, private bedrooms and a long hall.

The Japanese modern furniture, mostly upholstered in beige has just arrived after being held up with the dock strike. The Togos have furnished their private quarters with Western traditional furniture: a Chippendale sofa, a Queen Anne chair, leftovers from the old embassy.

The house has a permanent staff of eight, not counting the women who come in to do the very handsome and elaborate flower arrangements and the extra help for parties.

The building has, in all, 1.6 acres of floor space, and the lot is next door to the Swedish Embassy. The drive is down the steep hillside, and around a center planting area. Mrs. Togo, a great lover of nature, regrets the big old trees removed during construction and before she came to the embassy, but many more have been planted.

The late Japanese architect Isoya Yoshida was the designer, and Charles H. Tomkins of Washington was the contractor.The house was begun in 1973, and the Togos occupied it in August "because," as she explained while showing guests through the house last week, "otherwise things would never have been finished. So we just brought our beds and moved in."

Ise Togo, the daughter of Japan's foreign minister at the time of Pearl Harbor, first lived in Washington when she was 3 years old and her father was the embassy's first secretary here.

The building's long masonry facade, with only a story and a half at the front, looks like a fortified castle, with only narrow slits relieving the front. The architecture is recognizably Japanese, but in the glass-and-steel modern Japanese style, not the sort of low, human-scaled, modest, flexible architecture that inspired so much of American contemporary design.

Washington's newest palace, the Japanese embassy residence, represents Japan, the industrial giant, not the Japan of the old woodblock prints.