Saving those great old mansions nobody can afford anymore is one of the more conspicuous ways the large brigade of foreign embassies has contributed to life in Washington. But in recent years, with the supply of suitable mansions running low, foreign governments have been building their own.

The Republic of Indonesia has done both. In 1951, not long after its first ambassador arrived in the United States, the new nation purchased the Walsh-McLean house at 2020 Massachusetts Ave. NW, a fabulous eclectic pile designed by Henry Andersen of New York in the early 1900s for gold millionaire Thomas F. Walsh. Last month the ambassador and many office workers from the embassy moved into a new building next door, designed by The Architects Collaborative of Cambridge, Mass.

Outside and in, the contrast between old and new cuts like a knife. The undulating old mansion is a veritable pastry of ornament culled with skillful abandon from the Beaux Arts book of historical styles. The new building is a crisp exercise in modern art whose abstract, economical elegance speaks volumes about the vastly changed circumstances between then and now.

A visitor to the old building can almost hear the echoes of parties that were held in its extravagant spaces, smell the smoke from the cigars of the rich and powerful men who once gathered there, and see the ladies in their elegant gowns, descending the serpentine wings of Andersen's Y-shaped staircase, a tremendous focal point of the towering skylit central hall.

It is a tour-de-force interior, a piece of social and architectural history that the Indonesians thankfully have preserved, and it also is a bit much. "How the money went!" exclaimed Evalyn Walsh McLean in her book, "Father Struck It Rich," and you can feel that, too. One frequent visitor to the mansion in its heyday thought it to be "very good, very rich and very ugly," and she was right on all counts. Passing from the old world to the new through doors in the conservatory is a not unwelcome shock.

So, there is something to be said for TAC's design strategy, which operates with an almost stinging clarity on the Massachusetts Avenue side. Unfortunately, this rationale falls apart completely when you look at the building from its back side. Ordinarily this would be of scant importance, but here, because the back side faces on an active city street (the 2000 block of P), it is a major failing, not to say a rude affront.

In any case, along Massachusetts Avenue the eye turns with a certain relief from the profusion of curves and ornament on Andersen's building to the sharp simplicity of TAC's addition, which is basically an office cube tacked onto a curvilinear entrance hall.

The contrast works mainly because the architects of the new building decided to establish a clear hierarchy of values between the two structures. Their addition unmistakably bows to the massive older piece: Its surface of low-key blond bricks respectfully echoes the narrower, tan bricks of the Walsh-McLean house; the addition is politely set back from the street, making a sort of neutral platform from which the older building rises; and the "S" curve of the new entrance hall and pedestrian connector between the two structures picks up, in a streamlined way, the fin-de-sie cle undulations of Andersen's building.

At the same time the new fac,ade is no shrinking sister. Humility changes to pride with a businesslike snap: the sweeping curve of the entranceway is tucked into an office wall composition that is pure mirror-glass Mondrian. Ornament is dead here, for sure. Out of necessity, but perhaps not without a touch of irony, the architects make a "sculptural" statement out of an exhaust fan that looks as if it were picked up at an estate sale of parts from some long-gone ocean liner.

The special set of circumstances that makes the TAC strategy effective on the one side does not apply at all along the other. Along P Street the building is nothing more than an aggressively ill-fitting box.

One can understand the problem without admiring the solution. The odd-shaped, through-block site is common occurrence in Washington wherever the diagonal boulevards cut through the orthogonal street grid. So, when client and architects decided to set the building back from Massachusetts Avenue and the old embassy building, this meant, willy-nilly, that its main mass would be pushed onto P Street. This is fine. The four-story mass of the new structure adds to the building wall on the north side of the street. Furthermore, locating the entrance to a parking garage along this line was a programmatic necessity and even the use of mirror glass can be defended as energy-efficient.

Even so, the design of this fac,ade is remarkable for its hubris and for its lack of finesse. Mirror-glass fac,ades, it is said, can provide important esthetic dividends by reflecting the sky and nearby buildings. But here the device serves only to call attention to a building that doesn't deserve it. Along P Street, the TAC strategy produced an instant eyesore. In Cambridge, Mass., perhaps, it wouldn't have mattered so much: cacophonous contrast between modern buildings seems to be one of the rules of the game up there. But in Washington it doesn't do.