A proposed expansion of Seattle's downtown convention center would cost an estimated $766 million, according to early cost estimates.

A proposed expansion of Seattle’s downtown convention center would cost an estimated $766 million, according to early cost estimates released by backers of the project Monday.

But an existing King County hotel tax that backers hope will pay for the project should bring in more than enough to pay for the expansion — and even leave enough extra to contribute $75 million toward a KeyArena expansion being sought by Seattle Mayor Greg Nickels, according to convention-center estimates.

Nickels also has sought a portion of that tax to pay for part of the KeyArena renovation aimed at luring an NBA team to replace the departed Sonics.

Frank Finneran, chairman and chief executive of the Washington State Convention & Trade Center, told a legislative task force that the expansion would allow the convention center to attract business it now turns away, claiming, “We are losing more business than we are booking.”

The proposed expansion would be built one block to the northwest of the existing convention center, over Metro Transit’s Convention Place Station. It would double the amount of exhibit space available for the center and would include a ballroom and retail space.

The funding would come from the existing convention-center tax of 7 percent on hotel- and motel-room bills in Seattle. Hotels and motels in the rest of King County pay 2.8 percent. That tax currently is devoted to marketing and paying off the debt from the existing convention center.

Jim Brunner: 206-515-5628 or jbrunner@seattletimes.com