Alnwick, United Kingdom
Inspired by a survey's findings that one-third of children "aren't allowed to climb trees," the Duchess of Northumberland commissioned what would become one of the largest tree houses in the world. It was crafted with materials from sustainable resources and features rope bridges, walkways, decking, a restaurant, and educational facilities—all wheelchair accessible. And, don't worry, wood braces and concrete provide additional support. The 6,000-square-foot Alnwick Gardens Treehouse was built among a group of mature lime trees on the grounds of the Duchess' home, Alnwick Castle. The castle is as famous as the tree house: It was used as The Hogwarts School of Witchcraft and Wizardry in the first two Harry Potter films.