In Boulder's aptly named Wonderland Hill neighborhood, deer and even mountain lions occasionally come down from the woods to scout the domestic scene, but the most common wildlife sighting on the tree-lined streets is a profusion of toddlers in off-road strollers. To make space for the local baby boom, many older one-story homes have had their pops topped. When Rob Pyatt and Heather Kahn were ready to expand on their 900 square feet, however, their foundation couldn't support a second floor, so Pyatt, an architecture student with a green building background, devised an alternative. His box-shaped addition is the modern kid on the block, with distinctive corrugated-metal and wide-plank cladding. Behind the facade, uncommon materials share a common story with the neighborhood: Of design decisions driven by a desire to keep the next generation—and the planet—healthy and safe.