By Ian Wilhelm

To pay for the building of its new headquarters in downtown Seattle, Bill and Melinda Gates have donated $350-million to their foundation.

That gift is one of the biggest sums anybody has given to charity this year, according to The Chronicle’s data base of donations that have been announced to the public.

Still, the gift is a small drop compared with other donations the Gateses have made to the $31-billion organization. In 2004, for example, the couple pledged $3.3-billion, of which at least 70 percent has been been paid. (The foundation said the new gift is separate from the 2004 commitment.)

In total, foundation officials said the Gateses have poured $21-billion into the organization.

The $350-million will pay for construction costs at the new Bill & Melinda Gates Foundation headquarters and campus located across the street from the city’s famous Space Needle. The $500-million facility, which will include a 15,000-square-foot visitors center and several environmentally friendly features, is expected to be completed in the spring of 2011.

“Our new campus will bring our Seattle employees together into one location,” the foundation said in a statement. “Designed to foster collaboration among our program teams, the headquarters will also provide us the space to convene experts and advocates from around the world who are driving innovation in global health, development, and education to share knowledge and ideas.”