Architect Bjarke Ingels lays out his vision with a slide show as Smithsonian officials and architects announce major changes to several core buildings during a news conference in 2014. (Bill O'Leary/The Washington Post)

The National Capital Planning Commission voiced serious concerns Thursday about the Smithsonian Institution’s proposal to remake the 17-acre area around its Castle, saying that the organization needed to better explain why it wanted to excavate several stories below the historic building and why the plan mostly ignores the recently restored Arts and Industries Building.

“It’s causing great anxiety,” said Commissioner Peter May, associate regional director of the National Park Service. “If we put this on the anxiety spectrum, this is at the top.”

The public also is alarmed. An online petition protesting the potential demolition of the popular Enid A. Haupt Garden, another aspect of the Smithsonian’s master plan, has garnered more than 700 signatures.

Resistance to the sweeping plan has been growing since it was introduced in November 2014. Designed by architect Bjarke Ingels, the project involves five buildings — including two national historic landmarks — along the southern edge of the Mall from Seventh to 12th streets SW. It is projected to take 20 to 30 years to complete at a cost of about $2 billion. The first stage of the massive plan, the renovation of the Castle, is expected to begin in 2021.

Smithsonian officials say the changes would improve connections between above- and below-ground spaces and provide better visibility and access to the museums from the Mall. The plan also would add such visitor amenities as restaurants and bathrooms, protect the Castle from earthquakes and create shared mechanical systems and loading docks.

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The Smithsonian’s update to the NCPC, one of two agencies that must approve the project, did not include the Ingels rendering of a futuristic plaza that would replace the Enid A. Haupt Garden behind the Castle. Also noticeably absent was any mention of a controversial seismic strategy, known as base isolation, that would require major excavation around and below the Castle. Both of those proposals were criticized by the public when the plan was introduced.

Ann Trowbridge, the Smithsonian’s associate director of planning, brought up the renderings right off the bat, saying they were helpful to explore the plan but “don’t represent completed designs.”

Commission Chairman L. Preston Bryant asked whether the Smithsonian had changed course on its seismic strategy, apparently because the officials had not mentioned base isolation, technology that is used on the West Coast but is rare in this part of the country. Trowbridge said that the more traditional approach also was being considered but that a decision may not be made before the final version of the master plan is submitted.

“I don’t think most master plans include seismic solutions,” she said.

That did not satisfy Commissioner Mina Wright. “As long as it has been introduced, it needs to be more fully explored,” said Wright, who added that she is not against the strategy but is frustrated by the Smithsonian’s failure to explain why it is being considered. “I don’t think the case has yet been made. It feels like [using] an Uzi to kill a field mouse.”

Wright expressed similar frustration with the lack of information about the Arts and Industries Building, which has been closed since 2004. The Smithsonian spent $55 million to restore the national historic landmark, but the master plan does not discuss its use. Smithsonian officials have said they can’t include the building in the plan because Congress is considering it for the site of a Latino museum.

Again, Wright was not satisfied.

“Use this master plan to get a decision about [it],” she said. “You’re going to spend a bucket of money, and it feels vaguely negligent to let it sit there. It comprises such a big piece of the puzzle.”

NCPC’s concerns echo the frustration of preservationists and others who have been working with the Smithsonian for 15 months to determine how the project might adversely affect the historic properties. Wright and May are part of that group. (Their next meeting is Wednesday at 5 p.m. at the Castle and is open to the public.)

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While officials Thursday focused on seismic technology and mechanical systems, area residents are mostly concerned about possibly losing the garden. Sylvia Cabus of Washington started an online petition this week to “Save the Enid A. Haupt Garden.”

“We go there regularly, and it is a wonderful destination for my family,” Cabus said. “It is a happy surprise when you enter from the Mall side. It really is a jewel.”

Smithsonian officials maintain that they are planning to expand the gardens behind the Castle and that they will consider many options at a later date, a position that Undersecretary Albert Horvath repeated Thursday.

Cabus and others remain skeptical.

“The Smithsonian is trying to manage this in a PR way,” she said.