A model showing the new subterranean features where the Enid A. Haupt Garden now resides, as Smithsonian officials and architects announce major changes to several core buildings on Nov. 13. (Bill O'Leary/The Washington Post)

At Thursday’s news conference announcing a $2 billion reconfiguration of the Smithsonian Castle, architect Bjarke Ingels indulged a subtle sleight of hand. As he walked the audience through plans to connect the Castle with nearby museums and create a canted plaza with curving, upturned corners beckoning to visitors on the Mall, he showed a slide that also radically reconfigured a neighborhood that isn’t part of his design.

In place of the ugly bureaucratic architecture on the south side of Independence Avenue, Ingels showed an enticing world of smaller buildings and open streets. Gone was the U.S. Department of Energy’s Forrestal Building, which straddles 10th Street SW, visually blocking access from the Mall to the waterfront and creating an ugly, modernist barricade that seems to hem in the Gothic-Romanesque­ castle that has served as the Smithsonian’s longtime central office and its most cherished visual icon.

Where the hulking, oblong concrete box of the Forrestal once stood, the Bjarke Ingels Group (BIG) envisions a fantasy of new, glass-clad and more civically scaled architecture. It was a gorgeous nocturnal vision of Washington, twinkling with lights, full of pedestrian bustle and lively urban energies. The Forrestal Building is the Gordian knot of Southwest Washington; remove it, and everything starts to flow.

Although redeveloping the Forrestal complex is included in an ambitious master plan for a new Southwest “Ecodistrict” developed by the National Capital Planning Commission, no one seems to know how long that will take. The General Services Administration has begun to make some positive changes to properties it controls in that part of Washington, but the agency could not give even a tentative time line for removing or redeveloping the Forrestal Building.

And so among the more worrying concerns about the enormously complex plan announced Thursday is how dependent its success may be on the realization of an equally or more ambitious and perhaps decades-long plan to remake the federal landscape to the south of it.

A rendering of the National Museum of African Art entrance from the Mall. (Courtesy Smithsonian Institution)

The central visual element of BIG’s plan is a complete redesign of what is now known as the Enid A. Haupt Garden, which sits directly across Independence Avenue from the Forrestal. The garden would be replaced by a gently tilted plane of grass, sloping down toward the Castle. Surrounded by light wells, with two rakishly upturned corners on the north side, the plaza would frame a sunken entrance to the Independence Avenue face of the Castle building. The old pavilions that serve as entrances to the Sackler Gallery and African Art Museum would be gone, replaced by new entrances at the raised, curved corners of the tilted plaza.

In some ways, this recalls the angled plane of grass introduced by Diller Scofidio + Renfro into the modernist plaza of Lincoln Center in New York, and it serves a similar purpose. It creates underground space and access points for light without filling in a public plaza, and it introduces a bit of unexpected and fashionable eccentricity into a rigidly determined architectural ensemble. In New York, this playful confusion of architecture and landscape works; it is odd, and it delights.

But it’s not clear whether there is enough room in what is now the Castle’s charming back yard for this gesture to succeed. Dissonance is lovely, and Washington needs more of it; but the upturned, curved entry points also feel too tightly compressed, forced into the interstices between the 19th-century language of the Castle’s fanciful neo-medieval­ stonework, the polychrome brick ragtime of architect Adolf Cluss’s Arts and Industries building, and the dour, Renaissance-revival bulk of the Freer Gallery. Ingels’s curly-chip architectural flourish is the sort of thing one would like to see happen someplace in Washington, but is this the right place for it?

The slides showing a remade south side of Independence Avenue make more sense. The canted plaza will read differently if someday it is connected to a lighter, more airy and contemporary urbanism across the street. BIG has remade Washington to give its remade Smithsonian room to breathe. But if the SW Ecodistrict plan remains an unfulfilled fantasy, and the Forrestal Building haunts us for decades into the future, the Smithsonian plaza may feel oddly incongruous with everything that surrounds it.

These are initial reservations, not fully formed doubts. There are many things in this plan worth celebrating, if the money can be found and if funding it doesn’t distract the Smithsonian from larger priorities such as renovating other buildings, improving its programming and expanding its role in American intellectual life.

The master plan would remove a loading dock between the Sackler and Freer galleries, and a parking lot east of the Arts and Industries Building, which will eliminate two ugly disruptions as you travel along the campus’s east-west axis. It would deal with essential renovation and systems issues, and address what the Smithsonian says is a pressing need for a seismic retrofit of the Castle after the 2011 Virginia earthquake.

The addition of an underground space around the Hirshhorn’s Sculpture Garden is a simple, obvious and non-disruptive improvement to a museum that has been looking for more room as well. It would create light-filled galleries and connect visitors inside to the sculptures in the sunken garden on the Mall. The plan also calls for sinking the fountain inside the Hirshhorn’s circular courtyard — and this should be studied very carefully, because it could greatly affect the simple austerity of Gordon Bunshaft’s original design, and create a potentially cavernous space with the museum looming overhead. Reopening the underground connection between the museum and the sculpture garden — a subterranean passage beneath Jefferson Drive that was closed years ago — is a no-brainer.

A rendering of the Sackler Gallery entrance from the Mall. (Courtesy Smithsonian Institution)

The plan also calls for removing interior additions to the Castle’s Great Hall, and relocating the visitor amenities and retail that make it feel like a cluttered, second-rate airport bookstore. No one will miss the pavilion-like entry to the Dillon Ripley Center, a warren of underground spaces housing various Smithsonian events. But plans for removing the entry pavilions for the Sackler Gallery and African Art Museum will be more controversial. Demolishing them gives the architects more space to play with and would create a large, and more coherent, quadrangle. But the atrium of the Sackler is a successful museum portal — a quiet, contemplative buffer between the Mall and the underground galleries — and these existential portals play a significant, if undervalued, role in the museum experience.

That brings us back to the Enid A. Haupt Garden and the contemporary plaza that would obliterate it. On one of Ingels’s slides was what appeared to be a small black-and-white rendering of the gardens that framed the Smithsonian to the north in the 19th century, gardens that were removed when the Mall was laid out. These pleasure grounds were greatly missed when they, too, were destroyed to create a grander, more imposing greensward from the Lincoln Memorial to the Capitol. They were a local, social, human-scaled landscape, removed in favor of imposing a national vision of grandeur, progress and rationality.

Although it was opened in 1987, the Haupt Garden is a living memory of the gardens that were lost. It is a formal and old-fashioned space, pleasantly shut off from the wide-open, barren exposure of the Mall. A garden that invites private escape, intimacy and disconnection will become yet another public space to be “activated” and “programmed” with events. If it is lost, we are also likely to lose some of the little follies and miniature gardens it contains, including the sweet little Moongate Garden (near the Sackler) that recalls Chinese design, and the Fountain Garden (near the entrance to the African Art Museum).

Ingels’s plans for a stronger east-west axis also calls for piercing through the Mary Livingston Ripley Garden that divides the Hirshhorn from the Arts and Industries Building — one of the great, secluded, secret spaces in the mostly arid reaches of Monumental Washington.

Taken together, we see not just a plan to make the Smithsonian grounds more agreeable, and rational, but an attack on compartmentalized garden spaces in favor of open public plazas and axial flow, which may parallel the larger loss of private or hidden spaces as our society becomes an integrated nexus of entertainment, surveillance and security. If the Mall was a 20th-century overlay of modern ambition on a 19th-century park, the new plaza is like a 21st-century screen imposed on a garden; it will have to be “on” all the time, always playing something, always doing something to entertain us.

Now the hard part begins. There will be endless review, from myriad agencies and oversight groups, with historical preservationists playing an essential role in determining what is too cherished to be sacrificed. The engineering challenges were glossed over at the news conference, but creating new space underneath the Castle, and connecting it to the Great Hall above, will be a challenge, and expensive. Is it worth the cost? And will the carbon footprint of this vast construction project be offset by proposed gains in energy efficiency? And what of the Arts and Industries Building, which may or may not become the new Latino American Museum? Should we create new space when the use of existing space is undetermined?

The conversation must proceed from rational grounds, which means the architects should also stop showing a slide that presupposes major changes to the urban landscape that could be years, or decades, away. It is slick advertising, and it appeals to our desire to be free of the worst of the grim Federal architecture that faces the Smithsonian. But it stacks the deck in favor of the Smithsonian’s ambitious plan, which will need to be considered dispassionately and thoroughly, and be guided only by the facts, and reasonable assumptions about the glacial pace of the federal government.