The opening of the $18 million American wing of the Metropolitan Museum of Art here is the single most important event in American painting, furniture and decoration in the last half century. Antique prices, scholars' opinions, house and furniture styles, all will be affected strongly by the opening June 11.

The new three-story wing, 150,000 square feet, is roughly six times the size of the old building it wraps around. Without a doubt, the collection it holds is the most comprehensive collection of American arts.

The glory of the new wing is the garden sculpture court.

At the north end of the garden court, the 1822-24 neo-classical marble facade of the United States Branch Bank of Wall Street has unfortunately lost the rest of its building. bBut framed and softened by tall trees, the facade manages to hold itself up very well, looking cool, calm and classical.

Wait a minute. Are its pediments raised ever so slightly in surprise? Is it looking down its columns at the upstart at the other end?

No wonder. The exuberant ceramic capitals adorned with poppies in their most sinful and lucious bloom invite you into Louis Comfort Tiffany's loggia with promises of delicious delights.

The loggis is the essence of Art' Nouveau architecture: asymmetrical, flamboyant, not a little decadent. None of the four capitals are alike. Each represents a poppy at a different stage of bloom. Its designer was not one to do things twice the same way. They blossom atop two pairs of columns and half-columns supporting stepped arches. The lintel is tiled with iridescent blue glass.

The loggia was built in 1905 as the entrance to Tiffany's home, Laurelton Hall, on Long Island. Its original glass doors have been replaced by a wonderful stained glass window showing Oyster Bay framed by wisteria vines blossoming with luminous blue flowers. The panel was designed by Tiffany about 1905 for the New York house of William Skinner.

On either side of the loggia are a pair of cast-iron staircases, partly bronzed, designed by Louis H. Sullivan in 1893 for the Chicago Stock Exchange. The design shows Sullivan's close kinship with the Viennese Secession style, the transition between Art Nouveau and Art Moderne. The staircases lead to the balcony and other architectural survivors including a Frank Lloyd Wright window from the house designed for the Avery Coonleys (parents of Washington's Elizabeth Ferry Coonley).

Mounted on the east wall of the court are purple (numidian) marble caryatids, working ladies, supporting the mantel of a fireplace. Incredibly, the mantelpiece is a collaboration in marble, mosaic and wood by John LaFarge, George B. Post and Augustus St. Gaudens for the entry way of the Fifth Avenue Cornelieus Vanderbilt mansion.

In between the two great facades is the great 70-foot high Charles Engelhard Court, a glass-roofed paradise so lush you expect the tiger and the lamb to appear any minute. Tall trees, brilliant flower, lush greenery reflect in a dark pool. Glass roofs the court and curves down to form one entire wall, revealing the glories of Central Park. The garden is populated with statues, including that most chaste nude, Hiram Powers' "Calfornia," and works by Augustus Saint-Gaudens. The marble bodies define the word "statuesque." Many princes' palaces would envy such magnificence if not grandiosity.

The courtyard represents the architecture of its time, the late 20th century, in the same way the other facades represent theirs. Kevin Roche/John Dinkeloo and Associates, supervised by the Metropolitan's Arthur Rosenblatt, designed the new wing and the court; landscaping by Innocenti & Webel; Berry Tracy, curator-in-charge.

The two ends of the courtyard better than anything else, illustrate the difference between the old American Wing, closed now for five years or so, and the greatly enlarged wing (previewing to invited guests at celebrations beginning next week before the public opening June 11). They also indicate the span of the collection.

Before it was all finished, we walked through the new wing with curator Morrison Heckscher, amid the sort of hammering and sanding, you might expect to find as you finish up a new house. "The American Wing/A Guide," by Marshall B. Davidson, published in conjunction with the opening, is an invaluable companion to the collections.

The old wing's collections began in the Colonial period, going through the Federal to stop in 1825 -- in deference to the 1920s' view that nothing younger than 100 years old was antique.

The new wing will, when finished in 1981, continue the collection through the Revivals -- Greek, Rococo and Renaissance -- to Shaker simplicity, Tiffany's Art Nouveau, Frank Lloyd Wright's Mid-Western organic architecture and folk art. The project is so vast, it will be completed in three parts.

Opening now are the nine Joan Whitney Payson Galleries for 18th and 19th century paintings and sculpture; the Erving and Joyce Wolf special exhibition gallery (opening with drawings, watercolors and prints); 18 period rooms and 11 decorative arts galleries. The balcony, beginning this fall, will show 18th and 19th century silver, notable for the huge ceremonial pieces; pewter, glass including some notable Tiffany art glass; and ceramics, expecially Pennsylvania German redware. To come in 1981 are the folk art gallery, seven more period rooms and the study/storage area (accessible to the public as are the Met's Egyptian study collections).

The new Met Wing goes beyond the traditional display of room settings, the voyeur or glimpse-beyond-the-velvet rope approach, followed in the old wing. Galleries beginning with the earliest periods on the third floor and working down, will display choice furniture and decorative objects as the art works they are.

In this, the Metropolitan is following the lead of the small but splendid show of American furniture at the National Gallery of Art (closing regretably on July 4). Though it should be noted that unlike the National Gallery, the Metropolitan had departments of architecture and decorative arts, before so honoring paintings and sculpture. Even back in the 19th century, the Metropolitan acquired some contemporary works, including some of Tiffany's.

Today, recognition is growing that utilitarian objects don't have to be ancient Chinese to be high art.

The Hudson-Fulton exhibition of 1909 lead to the Met's acquisition of much Colonial furniture and silver and eventually to the original American wing. After it opened in 1924, all of a sudden, America felt grown up, as though culturally it could throw off the Old World or Early Mother styles which once had seemed the only right and proper ones.

The Metropolitan today has 1,000 paintings, 1,000 watercolors, 300 miniatures and 123 sculptures by American artists including Colonial portraits, Hudson River landscapes, American impressionists and folk artists. Winslow Homer and Thomas Eakins are well represented. Usually, paintings of artists born before 1876 will hang in the new wing with visits by the later artists. Some 300 paintings interspersed with 75 sculpture works are shown in this first phase.

You walk through the facade of the United States Branch Bank into a gallery of the Federal Period, with many pieces from Duncan Phyfe, as well as a Thomas Seymour sideboard from Boston. The Philadelphia neo-classical gallery has a wonderful 8-foot high secretary with the signature of the proud maker carved seven times.

Another is filled with 18th century Philadelphia Chippendale furniture, including a remarkable collection of high and low boy chests, ornamented with shells, leaves, flowers and other rococo naturalistic decorations. From the same period are the Goddard and Townsend chest and clock from Newport, R.I., in the 18th century furniture and decorative arts gallery.

The introduction to the early Colonial Period is in a gallery roofed with heavy rough-hewn trusses, reminders of the great halls of England, but here adapted from the Old Ship Meetinghouse at Hingham, Mass, built in 1681 and still (we can be thankful) serving its worshipers. In this room you can see the most important simple furniture of the period -- sunflower chests from Wethersfield in the Connecticut Valley; a marbelized trestle table, one of the earliest known; a highly figured, veneered desk in the William and Mary style. The Metropolitan's collection of early furniture is especially rich.

The 18 period rooms from the late 17th century to the early 19th, already installed, have furnishings original to the time, but the textiles, curtains, bedhangings and upholstery, are reproductions.

One of the best rooms looks familiar -- the 1792 ballroom from Gadsby's Tavern, with its musicians gallery, spirited away from its home. It's splendid to see it so well preserved and honored at the Metropolitan, but it seems sad that Gadsby's, now revived as a working tavern in Alexandria, has to do with a copy of its original paneling and woodwork.

As you walk through, time spins in front of you. A medieval 1660s low ceiling room with a massive fireplace has century oak and pine Massachusetts furniture including a marvelous oak chair table. A splendid 1710 parlor from Portsmouth, N.H., has a maple veneer high boy in the William and Mary taste and a Turkey "carpit" used as a covering for a gateleg table.

An 1810 Baltimore dining room boasts a painted floor cloth and some interesting Baltimore furniture incuding a card table with satinwood inlays. An 1811 Richmond room has solid mahogany woodwork, King of Prussia marble baseboards imported from Philadelphia, and later Duncan Phyfe furniture. The entrance hall to a 1765 manor house from Albany, N.Y., is covered with a handpainted wallpaper.

A pedimented doorway is carved to look like cut stone, with other ingenious ornaments from a 1760s Westfield, Mass., house, and a Pennsylvania German fireplace in a room with cupboards and boxes painted with decorations in that manner. The Van Rensselaer Hall of Albany, N.Y., is full of New York Chippendale furniture, including ruffle and tassel backed chairs. Remarkably, all the furnishings in the Verplanck room are from the same New York City house.

The music room from Wright's Francis W. Little House in Wayzata, Minn., will be installed in later phases, along with six other rooms including a 1882-84 McKim, Mead and White Colonial Revival living hall, and a 1830 Shaker Room.

The Metropolitan's new house of American treasures gives us great reasons to be proud of American craftsmanships: the well-turned leg, the cleverly crafted silver handle, the elegantly carved finial. How wonderful that such craftsmanship has been revived today with our quilters, silversmiths and woodcarvers.

The architectural artifacts arouse mixed feelings. It's glorious to admire Tiffany's delicious capitals, Sullivan's strong staircases, and Wright's innovative geometrics. But we weep for the burned Laurelton Hall, the demolished Chicago Stock Exchange and the dismembered Frank Lloyd Wright house.

The opening of the new Metropolitan wing is bound to add to the already great interest and prices in American furniture, paintings and decorative arts of all periods.

The recent block buster sales at auction houses including C. G. Sloan's and Weschler's here, Sotheby Parke, Bernet, Christie's and Phillips Gallery in New York emphasize the huge monetary value of American collections, often surpassing English antiques, from which many of the Colonial and Federal American pieces are derived.

In Washington, the extensive collection in the State Department's Diplomatic Reception rooms, and in the White House, both a monument to curator Clement Conger's tenacity, have largely come about in just a little over a decade. The White House recently has launched a campaign to establish a $16 million trust to buy more art and furnishings for the house and to maintain what is there.

The Smithsonian's Art and Industries building also has model rooms going up through the Victorian period.

The huge prices of genuine American antique furniture has led to a renewed interest in reproductions. At the recent furniture mart in High Point N.C., important furniture lines such as Baker and Thomlinson renewed their dedication to fine copies of traditional furniture. Most of these will soon be represented in Washington stores.