The University of Washington has unveiled an $850 million proposal to build a series of residence halls and apartments on its main Seattle campus to house more than 3,000 students. The school says the housing is needed to relieve crowding. The plan calls for eight new buildings, each up to six stories high, the first to be completed by the fall of 2011. Many would be built on what are now parking lots. The university also would renovate six of the seven main residence halls, while demolishing the seventh -- Mercer Hall, a less popular student residence on the west side of campus.