The panoramic view of the Mall from the balcony of the Hirshhorn Museum is at all times spectacular, but tomorrow evening, when the Smithsonian regents and guests step out onto that concrete platform before one of their triannual dinners, the prospect will include an island of pleasant surprises.

Gardeners and art handlers have been working like ants for several weeks to complete the renovation of the Hirshhorn sculpture garden in time for the regents' fete. They just made it. The last tree was planted yesterday and the last sculpture installed: Rodin's "Burghers of Calais," hauled across the Mall again after its splendid temporary visit to the East Building of the National Gallery of Art.

"It is a real shame that Joe won't be there to see it," lamented a museum employe, referring, of course, to donor Joseph H. Hirshhorn, who died 12 days ago. It is indeed likely that Hirshhorn would have enthused over the dramatic changes that have been made in the sunken outdoor garden to house the artworks that he often sentimentally called his "children."

The changes ostensibly were made in order to make the garden accessible to people in wheelchairs, but make no mistake, the transformation is much more. "We took advantage of the opportunity to correct what we felt were major problems with the space as it was," explains Hirshhorn director Abram Lerner.

What has transpired in that burrowed plot of land alongside the Mall is a rare combination of ingenious, sympathetic design and happy accident.

Lester Collins was the landscape architect chosen by Smithsonian secretary S. Dillon Ripley to confront the ticklish problem of revising architect Gordon Bunshaft's stark, cubic design. Collins did an admirable job, managing on a limited budget -- the entire renovation project cost $586,900 -- to retain Bunshaft's basic structural plan and yet to utterly transform the look and mood of the place.

"It's the difference between a striptease and walking into a roomful of naked women," quipped Hirshhorn deputy director Stephen Weil, comparing Collins' design to Bunshaft's. Ironically, the results make the place much more akin to the open, green setting of the Hirshhorn collections' former home in Greenwich, Conn., where the sculptures were installed before the big move to Washington seven years ago.

In effect, the Collins design is the third in a series dating back to Bunshaft's original scheme, conceived in the late '60s, to place the sculptures in a notorious trench, two football fields long, stretching across the greensward of the Mall. Fortunately, scarcely moments before construction was to begin, the architect was persuaded that whatever its other advantages or defects this "transverse axis" (as they called it) would treat the sculptures like pins in an outsized bowling alley.

Bunshaft's second design, the one that was built, turned the long trench into a broad sunken rectangle bordering the Mall's central stripe of green. In themselves, these changes of shape and location were significant improvements. The greensward sacred to Washington planners was not violated and, more importantly, the works of art were given room to breathe and play against one another under a vast dome of sky.

The parched severity of this rectilinear design was not without merit, but the appeal was more to the mind than to the senses, more theoretical than practical. Bunshaft clearly intended to achieve a marriage between his own outspoken brand of elegant modernism and the time-honored civility of the Japanese garden -- an effect similar to the wonderful stillness of the sunken marble garden Isamu Noguchi designed as a contemplative outdoor antechamber for Bunshaft's beautiful building at Yale University, the Beinecke Rare Book Library.

It didn't work. A handful of major obstacles defeated the scheme, not the least of which was the absence of Noguchi, whose subtle mind and steel temper would perhaps have been up to the task. Even Noguchi would have been hard-pressed, however. Time was short and so was money. The number of powerful voices with something divisive to say about the matter was, as usual in Washington, too large.

The biggest blocks upon which Bunshaft stumbled, however, were the sheer size of the place, the Versailles scale of the Mall and the blistering heat of Washington summers. The largest and best of the artworks withstood the challenge of Bunshaft's wide-open and nearly scaleless space, but overall the place offered too little relief for the eye and, save for the shade of that lonely willow tree, none at all for the body.

What was needed was a sense of inviting intimacy, which is precisely what Collins has managed to provide by breaking up the space in an orderly way with brick walkways that echo the basic retangular configuration of the plot, with a profusion of new trees and plantings and, above all, with thick carpets of grass. The latter was pure lagniappe: More faithful to Bunshaft's severe concept, Collins' original plan envisioned an elegant pattern of three-foot-square blocks of blue flagstone in place of the yellowish gravel that made the place a mini-Sahara from June through August.

The curse of cutbacks has plagued the Hirshhorn from its inception 15 years ago; the cylindrical building, for instance, was supposed to be sheathed in marble instead of exposed in concrete, and a below-ground restaurant with a view of the sculpture garden was eliminated from the plan. But the prohibitive cost of the flagstone -- $910,000 -- was a blessing. Grass is not only cheaper but in this case better. In addition, as Collins recalls, the decision to go with grass led to a general excitement about trees and plants with the result that banks of Japanese pines and thick "Gumpo" azaleas will replace the formal cascade of granite steps.

Collins started out with the idea that the wheelchair ramps would have to be an integral part of the space rather than cut-rate additions. The fact that his solution to the problem looks inevitable -- he tucked two long entrance ramps into the slope of the garden where it meets the Mall -- is proof of its inspired ingenuity. His plantings reinforce the identity of the garden as a welcoming urban park, particularly at the edges, with willow, cherry and dawn redwoods (Ripley's favorite) on the Mall side and hawthornes and ginkgos near Jefferson Drive.

Of course, the garden, which opens to the public Monday, is a park for art, and the crucial test is how commodiously it serves the sculpture. The new design passes this difficult test, too. The ramps, walkways, trees, shrubs and grass strike an apposite balance between active presence and neutral backdrop. The divisions of the space provide essential accents; artworks pop in and out of view as the spectator moves about the space, and yet they are given plenty of room when they need it. The installation of the sculpture, supervised by Joe Shannon, has many nice touches. Not only does the new design accommodate more pieces -- some 75 works as opposed to about 50 before -- but it also tends to be nicer to the many modest works of art in the collection.

There are, to be sure, certain dispiriting signs of a job done on the cheap -- jury-rigged bases for some sculptures, plain-pipe-rack railings interfering with the sweep of steps and ramps, cement surfaces that are jarringly bright in contrast to the soft-tinted aggregate and mulberry-colored bricks that set the tone for the place. But all in all, the greening of the Hirshhorn sculpture garden has transformed it into a jewel-like park within a park.