German design firm Baumraum custom-builds tree houses roughly the size of motorhomes. After taking into account the site—and health and future wellbeing of the surrounding trees and their roots—they prefabricate many of the houses. Of the 50 they’ve assembled around the world, prices (including construction and interior work) have ranged from approximately $20,000 to $167,000. That said, they’ve got a smaller, non-insulated option on offer for roughly $6,000. See incredible photographs of their work and read more at Dwell.