As a young child (or perhaps even an adult) who hasn’t dreamed of living in a tree house? Some structures are built on trees or hung from trees, but some unusual tree houses are even grown from trees or built right into a tree. Some people live in trees as a luxury, some to help save the environment and others out of tradition or necessity. Here are ten incredible tree house designs that range from functional to fanciful, sustainable to strange and affordable to incredibly expensive.