THE 60-ROOM Walsh-McLean mansion at 2020 Massachusetts Ave. NW, the centerpiece for today's Dupont Circle House Tour, may well be the most ornate mansion in Washington, and certainly the closest to an Art Nouveau design.

Outside, its undulating four-story-high walls have a Louis XVI limestone trim and a loggia with marble columns. Inside, the flaring Y-shaped three-story-high staircase with the two columned loggias is topped with a huge coved stained-glass skylight and surrounded by a series of 15-foot-high entertainment rooms, some of the largest and most elaborate in Washington.The mansion was built by Thomas F. Walsh in 1903 as his principal residence in a time when it was fashionable for millionaires to establish a seat in Washington. Harry Anderson, a New York City architect, designed the house, which cost about $835,000. Walsh, an immigrant from Ireland at 19, earned the money to build his house by discovering the Camp Bird Mine at Ourray, Colo. A piece of the gold ore from that mine is allegedly in the house's foundation.

The house has entertained Alice Roosevelt and King Albert of Belgium, among others. At one party, 325 guests quenched their thirsts with 480 quarts of champagne, 288 fifths of Scotch, 48 quarts of cocktails, 40 gallons of beer and 35 bottles of miscellaneous liquors, according to a New York Times account.

After Evalyn Walsh McLean bought the Hope diamond, said to carry a curse, a series of disasters hit her family and the house itself lost much of its glory. In 1951, the Indonesian government bought the mansion for $335,000 to use as a chancery.

The small parlor with the painted ceiling is crammed full of awkward office desks. The ballroom has been partitioned into a warren of offices. The organ doesn't work anymore. Daytime guests aren't even allowed to enter through perhaps the grandest foyer in Washington. Instead, they're funneled through the porte-coche re, the family entrance on the side. A stage has been built, rather jarring to the room's decoration and architecture, across the end of the drawing room. The conservatory is full of typewriters, not plants.

The house looks like a magnificent grande dame sent out to earn a living in a job to which her past life had not accustomed her.

But things may be looking up for the Walsh-McLean mansion. The Indonesian goverment is building a $1.3-million, 22,800-square-foot addition next door. Offices will take up 16,000 square feet. Another 6,800 square feet will be for parking.

After the building is completed in May, the Indonesian Embassy will be able to move many of its offices into the new building, leaving the first floor of the mansion free for entertaining and conferences. No one expects the Indonesians to restore the ballroom or the theater that the Walshes once installed on the fourth floor under the roof garden. The upper floors and the basement will continue to be offices.

The addition is two stories in the section fronting on Massachusetts Avenue, three stories in the middle of the block and, thanks to the slope of the land, four on P Street. The Massachusetts entrance is through a paved arrival court with a circular planting area, a fountain, and a circular turnaround. Still in question is the installation of an iron fence and gate on Massachusetts. On P Street, the garage entrance is softened by a fence.

The steel-framed addition will use gray glass and the same color of white brick as the original building.

"We tried to respect the old building," said James Burlage, partner-in-charge for the Architects Collaborative of Cambridge, Mass., designers of the addition. "We set the new addition as far back as we could, behind the mansion's rear wall, so as not to disturb the old mansion. The facade of the addition is curved, a whiplash curve if you please, to echo the lines of the mansion. The only place the addition touches is in the semicircular glass link between the two."

Joseph Handwerger of Washington, the associate architect on the project, explained: "The addition is a background building, it isn't intended to be assertive. We went before the Joint Committee on Landmarks four times to finally get their approval. And we eliminated some parking in the arrival court at the insistance of the neighbors."

The new building is actually in two sections: the S-curve front section with its swelling semicircle, which holds the lobby on the first floor and the ambassador's office on the second; and the glass-fronted rear building with an atrium and a garage entrance on P Street.

The curving section extends behind the passageway as a rather strange appendage.

The link between the buildings joins in a rather ungainly fashion at the conservatory, turning that room, one of the most handsome in Washington, into an entry hall. It might have been better to have let the new addition stand on its own feet. Still, most of us who are fond of the old mansion, can be grateful that nothing worse was done to it, and hope that the old house will soon be restored to its glory.

The House Tour

The Dupont Circle House Tour will open nine other houses today. Architect Charlotte Burk, on the Dupont committee, has researched the architectural history of the houses for a brochure.

* Burk calls 1400 21st St. NW "brick decadence" style. Mr. and Mrs. Richard Turner own the house, built in 1877-79 with Oriental motifs and stained-glass windows.

* Mr. and Mrs. George Wheeler own 2019 N St. NW, built by Christopher Thon in 1879-81 for his own family. The house is in a row with other houses of a Second Empire style, which Burk thinks is the finest residential example of the style in Washington. Thon was a co-founder of the Riggs Bank.

* The rusticated Seneca sandstone house at 2110 O St. NW was built in 1892 by Appleton P. Clarke Jr. in the chateauesque manner, one of five in a row. Gen. William L. Bramwell built the row and and sold them to his friends, staff officers of the War Department. The current owners, Thomas Zelanev and Jose Luis Vivas, found the oak woodwork intact and the original wall sconces in the attic. The owners have furnished the dining room with a magnificent Louis Majorelle Art Nouveau dining room set, bought by the Vivas family in 1895.

* The Victorian house at 2021 O St. NW was once one of the 45 row houses that together were called Hartnett Hall, a rooming house. The house is now owned by Michael and Patricia Gorman. With the help of Carol Lindenburg of Potomac Designs, they redecorated the house in the Victorian manner. Their art collection includes works by Susan Davis, Mark Leithauser and Fred McDuff. The master suite takes up the whole floor. Its bathing room has a reclining sofa and fireplace. Three of the houses were designed by architect Luis D. Meline, all in different styles, as was customary in that period.

* For 40 years, Alva Dawson has owned the Queen Anne Victorian at 2017 O St. NW, built in 1890, now in apartments.

* Gail Rogers and Robert Bruce Williams own 1337 21st St. NW, built in 1891 in the Richardson Romanesque style. Don Hawkins remodeled the house in 1980 to give portrait painter Williams a two-story-high studio.

* Hugh Newell Jacobsen designed the kitchen and greenhouse added to 1401 21st St. NW by the current owners, Rep. and Mrs. James Leach. The house, built in 1895, is Renaissance Revival. The dining room is octagonal.

* Two Edwardian apartment houses, the Leland at 2012 O St. NW and the West End at 1320 21st St. NW, were built in 1917-18. Both are now condominums. At the West End, the tour will go through Joyce Callahan and Dennis Sheehy's basement apartment with its small patio and private entrance.

Tickets ($8) for the 1-to-5 p.m. tour today are available at the Leland, 2012 O St. NW and the Indonesian Embassy, 2020 Massachusetts Ave. NW.