San Antonio Children's Museum officials unveiled a plan Thursday to build a $45 million complex north of downtown on Broadway that will nearly double the space for exhibits and improve accessibility for visitors.

Almost half of the project cost will be covered by a $20 million personal donation from Charles Butt, the chairman and CEO of H-E-B.

The museum will launch a fundraising campaign to cover the balance, with the city expected to kick in $3 million from next's year's $596 million bond program for public improvements at the site.

It's the largest single donation Butt ever has made, museum officials said.

Butt, the city's wealthiest resident, is known almost as much for his education philanthropy as for his chain of grocery stores and is considered one of the city's great supporters of public schools.

“San Antonio children are tops and they deserve a first-class children's museum,” Butt, who's currently out of the country, said in a written statement. “When properly done, children's museums contribute significantly to the creative development and education of our young people.

“Houston, Dallas and Indianapolis have particularly outstanding children's museums, which serve as learning centers for the early childhood set,” he said. “San Antonio is too large a city not to have one, and if we are committed to improving education, the time is now to create a newer, larger children's museum of national caliber accessible to all children in San Antonio.”

The museum used a quarter of Butt's donation to buy a defunct car dealership on a 5.4-acre lot near Broadway and Mulberry Avenue, across from Lion's Field. Vanessa Hurd, the museum's executive director, said construction is expected to start in two years and should be completed by 2015.

The museum's current building, at 305 E. Houston, is 40,000 square feet. The new museum will be 70,000 square feet and will include outdoor exhibit space, a cafe and on-site parking.

The museum plans to sell its Houston Street location.

A conceptual plan for the new museum shows an inviting, airy edifice stretching across the block on Broadway, wrapped with sidewalk plazas, front and rear parking and a side entrance for bus groups. But officials said the final design will be based on a master plan that will reflect the needs of exhibit spaces.

Of the $45 million cost, $3 million will be set aside in an endowment for operating expenses.

The new museum will be in a corridor that already houses the San Antonio Museum of Art, the San Antonio Zoo, the San Antonio Botanical Garden and the Witte Museum. The children's museum is the only institution to focus solely on children up to 10 years old, Hurd said.

Annual visits to the museum have risen over the years, reaching an estimated 180,000 this year. In the new space, Hurd said she expects to tally 400,000 or more annual visits.

Though museum leaders began discussing the need for a larger location well before residents coalesced behind Mayor Julián Castro's SA2020 initiative, the museum's mission aligns with several of the long-range plan's key points.

With an expanded and more accessible site, the children's museum will become a premier resource for early-childhood education, Hurd said.

“This is going to allow us to become an even greater resource, and one that really closely aligns with the city's priorities, specifically those outlined in SA2020,” she said. “We know that early-childhood education and literacy are critical for our children. SA2020 also talked about the importance of healthy families and SA2020 also talked about the importance of arts and culture, and those are three areas in which the children's museum is especially well aligned.”

The museum will focus on literacy, science, health and art programs. It's the most recent example of Castro's vision to get the entire community to contribute to his long-range planning project.

From the beginning, the mayor has said the direction of San Antonio could be improved only with community buy-in, and he's long called on nonprofits, private-sector business and the public to help.

“I commend Charles Butt for his generous and steadfast support of education in San Antonio and the state of Texas,” Castro, who was in Washington, said in a written statement. “By expanding the size and offerings of the children's museum, we can better leverage our creative assets in San Antonio to become one of the nation's most vibrant brainpower communities.”