Covered with 8 million banana-yellow tiles, the Rafael Viñoly-designed addition to the Brooklyn Children’s Museum looks like a shimmery spaceship that has just touched down among the brick and brownstone buildings of Crown Heights.

When the museum reopens in September, it will have doubled in size, to 102,000 square feet, and increased its annual capacity to 400,000 from 250,000. If all goes as planned, it will also be New York’s first LEED-certified “green” museum, with a geothermal heating and cooling system, floors made of sustainable bamboo, and state-of-the-art motion and carbon dioxide sensors that adjust lighting and ventilation based on how many people are in the museum at a given time.

“The goal was to serve more children and families,” the museum’s president, Carol Enseki, said of the expansion project, which began in 2003. (The museum stayed open during most of the construction, closing only last fall.) “Our space constricted us.”

In its previous architectural incarnation, the museum was also rather hidden. Since 1977, it was submerged under an earth berm, with only an antique trolley kiosk sticking out above ground. Now, with more than 90 feet of glass curtain wall, the museum looks out to the neighborhood and invites passersby to look in.

“The way it looks now better reflects the way we relate to the community,” Ms. Enseki said.

Founded in 1899, the museum is the oldest children’s museum in the country. By locating it in Crown Heights, the founders hoped to serve the thousands of immigrant families who had lately arrived in Brooklyn, Ms. Enseki said.

“There were 300,000 children in Brooklyn in 1899, and 90,000 of them were not able to attend school because of space and family issues,” she said. Given those circumstances, the museum was intended to provide educational opportunities to all Brooklyn families.

That mission is partly why the museum has never opted to move to an area more heavily frequented by tourists. “The question comes up in the life of any institution when you raise a lot of money,” Ms. Enseki said. “We’ve elected to stay firmly put in central Brooklyn.”

The museum is part of the Cultural Institutions Group — the group of 34 arts organizations, including the Metropolitan Museum of Art, Lincoln Center for the Performing Arts, and the Brooklyn Museum, that occupy city-owned land and receive a significant portion of their budget from the city.

For this project, the city provided $46 million in capital funding. The museum raised an additional $28 million — from corporations, foundations, and individuals, as well as state and federal sources — for the endowment and for new programming.

A visit late last week suggested that the museum’s exhibitions will be as visually exciting as Mr. Viñoly’s architecture. There is “World Brooklyn,” a permanent interactive exhibit in which children can learn about real Brooklyn businesses, from a Mexican bakery in Sunset Park to Spumoni Gardens pizza parlor in Bensonhurst, and “Neighborhood Nature,” which is about natural habitats in the borough.

There is also a large exhibition and activity area for children 5 years old and younger, which was designed by an artist couple, Tim Watkins and Carol May, who have also designed exhibitions for the Children’s Museum of Manhattan, the New York Transit Museum, and the New-York Historical Society. Mr. Watkins, who by training is a sculptor, designed elements such as the artful railings on a climbing structure, and a sculptural installation of pots and pans that serve as percussion instruments, while Ms. May painted murals, including a New York cityscape.

“Everything we ever learned or did gets incorporated in some way,” Ms. May said of their exhibition designs. “We try very much to make everything we do art.”

The expanded museum includes a cafeteria, which will have kosher options to accommodate the neighborhood’s sizable Hasidic population, and a rooftop theater. There is a gallery for objects from the museum’s collection, which include natural history specimens, such as the skeleton of a Asian elephant, and cultural materials, such as a historical collection of lunch boxes. As in the old museum, there is a greenhouse, a garden, and a terrarium for the museum’s 20-foot-long albino Burmese python, Fantasia.There are still some construction challenges to surmount before September: Among other things, the geothermal well is not yet functioning properly. It is supposed to pump water out of the ground, distribute it around the building — thereby reducing heating and cooling costs — and then return the water to the ground. In early tests, everything has gone right until the last step, when the water pooled instead of being reabsorbed.

“The Queens Botanical Garden has the same well system, and we hear they’ve had the same problem,” the museum’s public relations manager, Libby Pokel, said.

Ms. Enseki, meanwhile, is eager to see children take possession of the new building. Early on in construction, while the site was being excavated, she said, one of the staff members came with her 3-year-old son. Seeing the digging, the child asked: “Where are they taking my museum?”

“We’re hoping, even with this expansion, to really maintain that feeling,” Ms. Enseki said.