When is an old country home a "Treasure House of Britain"? When it has "architectural and historic distinction," according to a spokesman for the National Trust, the charitable institution that owns and administers much of Britain's architectural patrimony.

More specifically, the house becomes a treasure when it is "substantial enough, big enough, to be able to support our idea of country-home life -- servants, stables, an estate over which one can enjoy oneself in sport. Land. A garden.

"And, um, money. Obviously, there must be a bit of loot somewhere to keep it going."

There are hundreds of estates so classified in Great Britain, dating from the 16th through the 19th centuries. Inside, they house some of the most distinctive objects of art in the country -- from paintings and sculpture of all periods and places to priceless period furnishings. With the opening Nov. 3 of the "Treasure Houses of Britain" exhibition at Washington's National Gallery, a selection of these objects will go on view in the United States for the first time.

The homes and their contents are the traditional repositories of many a family fortune, yet their maintenance has become an expense that even bloodline aristocrats find hard to afford these days. At least 200 of the houses now belong to the National Trust, turned over by inheritors who lacked enough loot to keep them. Some of the rest, like the Duke of Marlborough's Blenheim Palace, have been converted into private tourist attractions by their still-resident owners.

What the British upper class has lost in privacy, however, the less favored have gained in the opportunity to view what was long a closed world.

National Trust specialists have spent four years touring houses -- their own, those opened to the public by their owners, and even those still maintained as totally private residences -- to select the best bits of art and furniture available.

Some made offers the specialists were forced to turn down -- an ancestral stuffed bird collection, in one case. "It comes to the problem of personal priority," the spokesman explained. "One man's junk is another man's treasure."

But with the more than 700 pieces selected for the show, there was a happy meeting of mind and taste. The Duchess of Devonshire laughingly complained that she had lent so many things to the exhibit from her Chatsworth estate that she hadn't a chair left to sit on. National Gallery Director J. Carter Brown noted, however, that he spent a weekend at Chatsworth recently and "didn't notice a thing missing."

Amid the pomp that will attend the exhibition's first week are a visit by the Prince and Princess of Wales, accompanied by officials of the National Trust and the British government as well as a number of the private owners of the treasure houses. There will be a White House dinner and a Washington gala jointly sponsored by British Caledonian Airways and the Savoy Group of hotels here, which is sending over four chefs and even some of its own "vital ingredients" (e.g. smoked salmon) to be prepared "similar in style to that served at The Savoy Hotels in London."

All involved hope that the display will deepen American appreciation not only of the works of art themselves, but of a longstanding and much-beloved British institution -- the way of life of the upper class.

On the British end, there are some additional goals. The owners of the exhibits, including the National Trust, are looking to the show to focus domestic attention and influence on an ongoing campaign for more government money to be spent on the arts. They also want legislative changes in death duties and inheritance taxes that would enable those still holding private collections to keep them in their natural, country-home habitats.

Over the short term, sponsors and participants here are looking to the exhibition -- and the sparkling events surrounding its opening -- to inspire more Americans to come and see them in Britain. "We entered into it with a warm feeling that hopefully has some commercial rewards," said Victor Emery, executive director of the Savoy Group.

Tourist spending is a substantial part of Britain's income, and American tourists spend the most of all. In 1984, they left nearly $1.5 billion here, a 31 percent increase over the year before. Estimated totals for 1985 are more than 20 percent higher again.

Some of that money is from admission charges to the treasure houses. According to Brown, one of the most difficult parts of arranging the exhibition was its timing -- no one here wanted the exhibits to fly to Washington before the close of this year's tourist season, and all insisted they be back promptly at Easter for the opening of next year's.

In fact, admission charges are only a small portion of the total revenue of the National Trust holdings. Most comes from government, private endowments and contributions by patrons, who pay as little as $18 for yearly membership. Individuals in the United States can become patrons of the Royal Oak Foundation, the trust's American branch, for an annual $25, or founding members for $5,000. The trust hopes that the exhibition will encourage U.S. Anglophiles to back up their cultural preference with more and bigger contributions.

The Savoy Group, which includes London's prestigious and pricey Savoy, Claridge's, Berkeley and Connaught hotels, has set its sights even higher. "We hope to lure people to this country to see the treasure houses," said Savoy Hotel Managing Director G.R.C. Shepard, "and, I must admit, try to persuade them to stay in one of our hotels. And hopefully fly on British Caledonian."

Americans not among the lucky few on guest lists for the exhibition's gala dinners may take some comfort in the possibility of inclusion on a larger list: 80,000 people whom the Savoy considers "up-market" enough to get an invitation to a treasure house tour next year.

Designed to coincide with the exhibition opening, the mailings will go out to those described by Executive Director Emery as having "the extra time to dwell on the quality of life . . . people who don't want hassles."

The package tour, part of an "invitation" issued in the name of the National Trust, includes a British Caledonian round-trip flight, six nights in a Savoy hotel, a chauffeur-driven Jaguar for day trips to the treasure houses and a year's free membership in the Royal Oak Foundation. With first-class air travel, the inclusive price from New York (per person, double occupancy, excluding meals) comes to $5,559.

"It's done in a stylish way," Emery said. "We don't enter into anything unless it's stylishly done."