The best example we've seen of these "adult" treehouse apartments is in Turin, Italy, where one apartment building stands out like a greenthumb. Designed by architect Luciano Pia, the apartment building is called 25 Verde (which translates to "25 Green"). Standing five stories high with 63 rooms, the building lives up to its moniker and looks like a giant, beautiful forest.
According to the property's website, Pia integrated about 150 trees into 25 Verde's eco-friendly design (40 in the courtyard alone), which means less pollution and less noise for residents. The trees absorb harmful toxins put into the air by street cars and even create a "microclimate" during the seasons -- making the apartments cooler during the summer and warmer during the winter. Even the walls "breathe" at 25 Verde, as ventilated walls allow sunlight to stream in during the day time. Tree foliage provides protection from the sun when it gets too harsh. More photos at the source!