A financing plan for the next expansion of the San Diego Convention Center leaves 25 percent of the cost unfunded, according to a report delivered Tuesday to the City Council.The plan, announced by Mayor Jerry Sanders last week, covers 75 percent of the debt service on the $550 million expansion with a 3 percent levy on each room night for downtown hotels, a 2 percent hike for hotels in Mission Valley and around Mission Bay, and a 1 percent charge for more outlying facilities.Sanders estimated the assessments would generate $28-$30 million annually.The mayor said he wanted the businesses that would benefit from the expansion to pay for it.Steve Cushman, a Convention Center board member, said he is working with the Port of San Diego, area restaurants, shops and taxi firms to raise the rest of the money. He said the port contributed a combined $301 million to previous expansion projects.Councilman Carl DeMaio said the port better "step up big-time."There was no disagreement on the need for expansion.Cushman said San Diego is losing convention business because the center is too small. For example, the summertime Comic-Con International nearly left its home city after being wooed by Anaheim, but agreed to stay for a couple of more years."It's easy to talk about Comic-Con, but there's lots of other conventions that are out-growing our facility," Cushman said.The project would increase the size of the building by 961,187 square feet, to more than 2.75 million square feet. The floor space of meeting rooms and ballrooms would double under the plan.Charles Black, who is leading the expansion project, said the center would have the largest contiguous exhibit hall on the West Coast and an 80,000-square-foot ballroom with large windows facing San Diego Bay.He said a consultant's study showed a bigger center could book an additional 25 major events each year, bringing an extra 247,000 visitors to San Diego, which translates to 657,000 extra hotel room nights worth $121 million annually, Black said.According to Cushman, the plan to assess hotels to pay off construction bonds will need to be approved by the hoteliers, a process that could take as long as nine months. The financing plan would then be brought back to the City Council for approval, he said.No action was taken Tuesday.