For the designers and builders of the new home of the Bill & Melinda Gates Foundation, nothing about the project is small.

From its million-gallon rainwater tank to its 53,000 cubic yards of concrete—more than was poured to create Seattle's old Kingdome—the $500 million campus for the world's largest foundation is a project of immense proportions, and implications.

For those creating the headquarters, the biggest part is contributing—in their own way—to the foundation's expansive mission. "We all get to play a role in something much, much bigger. It's supremely motivational," says Steve McConnell, managing partner with Seattle's NBBJ Architects, which designed the foundation's new home.

The 12-acre campus next to Seattle Center will need more steel than the Eiffel Tower.

It's as if general contractor Sellen Construction Co. had been hired to build a new college in the heart of Seattle.

But big only begins to describe the new headquarters of a foundation that aims to be felt around the planet and whose stated mission is to ensure that all lives have equal value.

At the construction site, where workers are laboring under six-day, 56-hour weeks, "I think there is a sense that maybe it is the most important building they will ever work on," says Sellen President Scott Redman.

"People understand that they are working on something that is very important," he says. "There is nobody out there that doesn't get it."

It's the Seattle-based contractor's largest project ever, Redman says. Already, the site bustles with 270 workers, most of them Sellen employees. By the time construction hits its peak a year from now, the site will employ about 500.

NBBJ's McConnell says a goal of the design is what he calls "workplace serendipity," a connected workplace that facilitates better interaction.

That could help the foundation make even more out of its already immense resource base built on Bill Gates' Microsoft billions.

"High performance work is an objective," McConnell says. Currently, employees are housed in five offices near the new campus.

The foundation's new home was designed to better connect those who work inside its spaces and to "engage with the surrounding community," McConnell says.

Two boomerang-shaped buildings will dominate the campus. They will house about 600,000 square feet of office space, stand six stories high, and feature slim corridors with plenty of glass to soak up daylight.

The two buildings evoke an image, according to the architect, of open arms.

"This is a very iconic symbol of reaching out to the world," McConnell says.

The buildings will be connected by a centralized welcome center that will serve as the foundation's front door.

The northernmost of the two buildings will feature a four-story glass atrium, as well as a convening center adjacent to Fifth Avenue.

The southern building will sit next to the site's primary garage, a partnership with the city that will house employees and Seattle Center visitor parking. On top of the garage is a 1.5-acre living green roof.

Along the garage will be a 14,000-square-foot visitor center designed by Seattle-based Olson Sundberg Kundig Allen Architects that is meant to display the foundation's work.

In the center of the campus will be a garden and landscaping by Gustafson Guthrie Nichol Ltd. of Seattle.

The project's second phase calls for a third boomerang-shaped building on the far eastern side of the site. That building will be developed later to accommodate future employees, according to the Gates Foundation.

An underground tank capable of holding up to 1 million gallons will store rainwater that will be recycled, primarily for irrigation and toilet systems. That will reduce the headquarters' city water use by 70 percent.

The entire project is designed to achieve LEED Gold energy efficiency certification for Leadership in Energy and Environmental Design.

"We have been challenged by our client at every step to do things smart and to help further their mission," McConnell says.

To be sure, there are very few comparable projects, says McConnell. Some sources of inspiration for interior designs that NBBJ turned to were the Wellcome Trust headquarters in London and the corporate headquarters of Genzyme Corp. outside Boston.

Both buildings feature large, open spaces with maximized daylight and spaces that lend themselves to employee interaction.

This summer, the first batches of more than 9,000 tons of steel are being placed by the site's three tower cranes—more steel than in the Eiffel Tower in Paris and more structural steel than in the Gateway Arch in St. Louis.

Enough timber will be used in the massive project to build a 5-foot-wide boardwalk all the way around Seattle's Green Lake.

Even with such daunting statistics, it may be difficult, Redman says, to grasp the significance and scale of the project just yet. Just as it is difficult to grasp the scope of the Gates Foundation's work in education, health, and global development.

But once the campus takes shape, and crowds of visitors to the adjacent Seattle Center stop to survey the scene, recognition will likely grow.

"This is going to be one of those places that is not only well recognized in Seattle," Redman says, "but around the world."