Extra maids have been hired, a special dessert has been concocted, and closets have been cleared for Diana's clothes. Royal company is coming to the British Embassy today and everything is in pristine order.

Except Delilah.

"She's shedding like mad," says Lady Marjory Wright, sitting in the drawing room and leaning down to pick a few golden hairs off her dark stockings. The source of these hairs, Delilah, Lady Wright's ever-present companion and dog-about-the-embassy, reclines devotedly at her mistress' feet.

It's not Diana's dresses the Golden Labrador might rub against. "It's his trousers," says Lady Wright, chuckling. The possibility of dog hair on Charles' trousers will not get Delilah banished, however. "She's part of the family."

When Charles and Diana are deposited by helicopter near the doorstep of the British Embassy today, they will be accompanied by an entourage of 18, and a passel of luggage that has been politely estimated as "a lot" and more bluntly as "several tons."

The house will be full -- "packed to the ceiling," says Lady Wright, who along with her husband Sir Oliver Wright, the British ambassador to the United States, will be Charles and Diana's hosts and among their constant companions.

Despite the hoopla surrounding the visit and the size of the royal party, British Embassy officials insist, with a touch of smugness, that all has not been fuss and pandemonium in anticipation.

"We're geared to it," says Lady Wright. "There are many royalty."

Still, they don't begrudge Americans their royal fascination. "I think we'd be very disappointed if people weren't so interested and thrilled at the visit," says Lady Wright. "We're just sorry that other people haven't got royal families."

The embassy preparation, which began a year ago with conversations with Buckingham Palace officials, has required much more than simply stocking up on extra ironing boards (which they did).

"This is an extra special visit," Lady Wright says. "Diana has never been to the States. She's a fairy-tale princess to look at, but she's very serious, too. She does marvelous work in the U.K., visiting children's homes, old people's homes, especially hospices . . . She's becoming very interested in the drug problem, too."

The entire second floor of bedroom suites at the residence, plus new guest apartments in the chancery, will be occupied by the royals and their entourage, with Charles and Diana taking over two bedrooms, a sitting room and two baths.

The royal couple's suite is cloistered at one end of the second floor and that part is blocked off by a fabric-covered wood screen with a door in the middle. One bedroom and bath and the sitting room were decorated by Laura Ashley, and another bedroom and bath were done by Jean Monro. In the Ashley room -- Diana's -- there are two twin beds pushed together. Upon her arrival there will be a large bouquet of flowers in the fireplace. Charles' room has two single beds as well as a chaise.

In the suite is a combination safe for valuables such as Diana's jewels. The lock will be scrambled and changed and the new code given to Diana's lady-in-waiting, Anne Beckwith-Smith. "You don't want to bother royalty," says embassy butler John Lightfoot.

Royal staff who cannot be accommodated by the embassy will be put up at the Embassy Row Hotel.

In their embassy rooms, guests will find Crabtree & Evelyn soaps and toilet waters, as well as fresh flowers, some of which will be supplied by landscape designer Sir John Thouron of Unionville, Pa. He has also provided the embassy with plants and is invited to tomorrow night's black-tie dinner there. "Usually we have garden flowers," says Lady Wright. "I just hope we might still be having roses." The White House gardener, who is friends with the British Embassy gardener, supplied some extra plants.

When Charles and Diana have to get around town, they'll be chauffeured in the ambassador's silver Rolls-Royce in a motorcade. (The Wrights will rely on either a loaned Rolls or Lady Wright's white Jaguar.)

For reading in their rooms there will be The Washingtonian, Vogue, Harper's and Andy Warhol's Interview magazine. For entertainment, Charles and Diana will get a television set and a videocassette recorder in their sitting room.

Diana has requested neither time for shopping nor exercising, two activities she is known to enjoy. "She'll get lots of exercise running around Washington," laughs Lady Wright.

All the rooms will have fresh fruit, biscuits, cheeses and jellybeans. There are soft drinks if they get thirsty, and Lady Wright will also supply her favorite carrot and celery juice. "I don't know whether they will like it, but I find it very healthy," she says.

The prince will bring his own Malvern Spa water.

The household staff promises service to the royal party that is anything but nonchalant.

"I have two rules here that I tell the staff," says butler Lightfoot, R.V.M. (Royal Victorian Medal), who heads a full-time household staff that numbers 18. "They know it because this is our seventh royal visit and they've heard me like a broken record: You must not speak unless you are spoken to, and when you are in the dining room, of course, you don't stare at the royal family . . . It's not done intentionally. It's done out of curiosity and admiration, of course."

And generally, the staff aims to make itself invisible.

"You know, when we hear them come down the staircase we all dash out of the way," says Lightfoot, "because what we want their highnesses to feel is that this is their home for that three days, and we want it to be as quiet and as homey as possible. There'll be service, but we'll do it like it's magic -- like no one's around."

For example, all the furniture that must be moved out of the ballroom, to make way for tables for tomorrow night's dinner, will be carted out while the prince and princess are having lunch at the Mellons' Middleburg estate in the afternoon.

Lightfoot, who has worked at the British Embassy for 11 years, is a former butler to the Aga Khan and was once a house manager of an exclusive gentlemen sheep-shearers club in Australia. Ruddy-cheeked and soft-voiced, he wears a crisp, impeccably tailored charcoal coat and striped trousers, and addresses Lady Wright as "M'Lady."

"When we have house guests, we're up at 6:30 and we probably work until 12 at night," he says. "You snatch an hour off in the afternoon for a cup of tea and a rest."

Lightfoot doesn't expect to get much more than a glimpse of the royal couple -- at the dinner tomorrow night, perhaps.

"We'll probably see him depart but he'll be surrounded by an entourage . . . The meals that we carry upstairs will be served in the suite by the butler and valet and the maid will serve the princess. So we won't be in direct contact with them."

Embassy staff will take care of the royal couple's rooms. "Our girls will make their beds and clean their suites," says Lightfoot. The royal staff will get similar treatment. "We'll wash their shirts and clean their shoes, so that when being attendants in the royal party, they don't have surplus duties to carry out. We'll feed them and wait on them and there'll be an extra staff to make their beds."

Most of the personal care-taking of Charles and Diana will be done by the staff accompanying them. Diana is bringing two maids -- or dressers -- who will serve her breakfast as well as care for and put out her clothes. Neither of them will have such cleaning chores as doing floors or washing bathrooms. "We'll have all that done for them," says Lightfoot.

As for the Massachusetts Avenue embassy itself, doorknobs are being polished and paint on the building is being examined. The pale lemon and peach silk ballroom draperies that drooped in the Washington humidity have been "crisped" up and made more bouffant -- they are intended to resemble full ballroom gowns.

"The silver is always polished," says Lady Wright. "In fact, I feel it's going to be worn out one day with the amount of polishing that's been done. It's a wonder there's any engraving left on it."

Five of the entourage, including lady-in-waiting Beckwith-Smith and the queen's press secretary, Michael Shea, arrived in Washington earlier this week.

Among others expected in the royal entourage are Diana's hairdresser, Charles' valet, a butler, a baggage master, several British police officers (who will work with local authorities) and a doctor, Naval Sgt. Cmdr. Ian Jenkins.

With Charles will be two private secretaries, Sir John Riddell -- a longtime London banker -- and David Roycroft, a member of the British diplomatic service on loan to the palace. "On a grand scale, they're Donald Regan to Ronald Reagan," explains Andrew Burns, press counselor at the British Embassy.

Also included is Charles' equerry, Lt. Cmdr. Peter Eberle, whom Lightfoot describes as a "sort of gentleman-in-waiting. He knows where they're going, who the people are, who to shake hands with."

Says Burns, "These are the people who are at hand to help and provide assistance the royal couple may need in public engagements. They handle messages to be taken and carried. You can't expect the princess to drop everything and go to the phone."

The British address the couple as "Your Royal Highnesses" upon first meeting, and in second reference, "Sir" or "Ma'am." "British women always curtsy to royalty," says Lady Wright, "and the men bow from the waist. You're bowing to the idea of the royal family and the heritage."

Lady Wright will follow that tradition. "What you do is give one deep curtsy at the beginning of the day and certainly one when you say good night," she says. "And then in public, if she passes, I curtsy."

However, she adds, "This is not a rule for citizens of the States. If you like to curtsy, that's very nice. That's up to individual taste."

Charles and Diana's food tastes, which are highly publicized -- he doesn't eat a lot of meat; she doesn't a eat a lot of anything -- will, of course, be taken into account in the embassy's planning of meals.

"I'm going to give them poached salmon on one lunch because they're fish eaters," says Lady Wright. With that will be a strawberry mousse. (Charles doesn't like chocolate.) Another lunch may feature red snapper. "I think it will be nice to have a local fish," Lady Wright says.

The royal party will be served anything they desire for breakfast. Along with copies of The Washington Post and The New York Times, breakfast will be brought to their rooms on trays. "Which I think is a very civilized way," says Lady Wright.

"You never go to royalty and ask what time they want for breakfast," says Lightfoot. "You go to the secretary."

No one in the embassy household seems worried about -- or prepared for -- Charles and Diana craving a midnight snack. "Thank you for warning me," Lady Wright laughs, when asked about contingency plans. "Well, we could always have smoked salmon sandwiches ready. That's the sort of thing one finds out from their ladies-in-waiting -- what they're likely to want when they come in. It will be very unlikely, because they're light eaters . . . People usually want to avoid eating if they can when they've been in the public eye for a long time. You have to eat three or four meals a day."

However, if the royal couple should get a yen for scrambled eggs at 3 o'clock in the morning, they simply ring down to the pantry. "You wouldn't be very popular, but you'd get it," Lady Wright says.

Lady Wright, who plans the menu with the chef, refuses to divulge either the guest list or the menu for the formal embassy dinner tomorrow night in honor of the couple.

"If everybody knows what they're going to eat, it's a bit boring," she says.

When the furniture is cleared from the ballroom to prepare for dinner, there will be tables of 10 set with lace cloth over deep peach cloths. As is her habit, Delilah will probably wander in and lie in the middle of the room. "She always comes to dinner parties," Lady Wright says, "but she prefers receptions, because she gets little bits of food."