Littleton, N.H., a town of 5,965 in the White Mountains, is being touted by some urban planners as a model of cutting-edge smart-growth practices. For starters, the town requires that every existing building be filled up before new ones are built. And citizens are invited to participate in town hall-style meetings to pore over blueprints, talk about traffic, and make planning decisions large and small. This type of citizen-based planning, which has been going on in Portland, Ore., for 30 years, is now spreading to other towns and cities around the U.S., including Boston, Mass., Chattanooga, Tenn., and Rochester, N.Y. When Rochester recently created neighborhood task forces and asked citizens for planning recommendations, it got many suggestions and tried to incorporate them into the city’s master plan.