Baumraum tree house

These Genre-Busting Tree Houses From Germany Start at $6K3/19

"The interior and exterior surfaces can be left untreated or oiled, waxed, varnished, or painted," says Wenning. "There is a wide range of ecologically harmless products available. The interiors can also consist of natural insulation materials such as wood fiber boards, sheep wool mats, or coconut fibers."