It's a massive project taking shape during a steep decline in real estate development and commercial property values.

The Bill & Melinda Gates Foundation's new 900,000-square-foot headquarters, comprised initially of two six-story, boomerang-shaped buildings on 12 acres near the Seattle Center, is scheduled to be finished in April 2011 at an estimated cost of $500 million.

Update: The WSJ's Robert Frank asks: Does a charitable foundation need a $500 million complex?

The Gateses said Friday they are making a $350 million payment of personal funds into their foundation's $34 billion endowment for construction costs. The couple made the one-time payment to distinguish money for the campus from money they have given for grants.

The foundation purchased the parcel of land from the City of Seattle for about $50 million.

The construction project has been going on more than a year and is now about 40 percent complete. Here's a view of it from a live Web cam. It will house the foundation's nearly 800 employees, now working in five locations, and an 11,000-square-foot visitor center.

At the heart of the campus is an atrium six stories high that is completely open and enclosed by glass windows.

On the site at 500 Fifth Ave. N., 400 workers are busy welding steel, pouring concrete, operating now three cranes and reinforcing an underground sewer line. The building takes close to 7,000 tons of steel for the structure and more to reinforce the 67,000 yards of concrete. The project is being led by Sellen Construction based on a design by NBBJ architects.

Green building features include a living roof on the parking garage and a million-gallon rainwater storage tank to reduce water use. The project is aiming for a Gold rating in LEED Certification, an environmental building standard.

Taking a look at some other recent developments, the non-profit Mercy Corps completed its new global headquarters in Portland, transforming and expanding a historic downtown building at a cost of $37 million.

For its new headquarters in a building complex now under construction in South Lake Union, Amazon.com signed a deal to lease about 800,000 square feet for about $700 million, with an option to double that.

However, a recent national report predicted that the recession and bank troubles will continue to weigh down the Seattle market next year, with WaMu's collapse and new but mostly unoccupied office towers combining to push the downtown office-vacancy rate above 20 percent.

Northwestern Mutual bought the WaMu Center tower from JPMorgan Chase for $115 million, less than one-third of what it cost to build.

The Gates Foundation's headquarters is the biggest project in Brian Duke's 27 years at Sellen Construction, where he is senior superintendent.

The design and communications effort needed to pull it off is huge, he said.

Cranes operate in close proximity to high-voltage power lines. When winds are above 20 miles per hour, the cranes have to stop, which could slow progress over the winter. In the record heat this summer, temperatures on the steel decking reached 120 degrees.

One challenge was removing contaminated soil -- 600,000 tons of it, load by load. The soil and groundwater were contaminated from decades of fuel storage and vehicle maintenance.

Workers also had to rebuild part of a live sewer main in the middle of the project, he said. First they had to demolish an old brick manhole from the early 1900s, being careful not to damage the line, which runs underneath Republican and serves the South Lake Union neighborhood.

Duke said he draws inspiration from the foundation's charitable aims. "It makes it easier to come to work," he said. "Your job isn't just a construction worker; it has some meaning."