Designed by visionary Austrian artist Friedenreich Hundertwasser and built by architects Joseph Krawina and Peter Pelikan, Hundertwasserhaus is one of the most unusual residential buildings in Austria (52 apartments and 19 patios), one of European pioneer projects in regards to green roofs, “tree tenants”, “humus toilet”. Natural features as a roof covered with earth and grass, large trees growing from inside the rooms with limbs extending from windows, are incorporated in the structure. More than 250 trees and shrubs on the balconies and roof terraces make the Hundertwasserhaus a green oasis in the heart of the city. The grass and forest areas of the house amount to more than 100 per cent of the ground plan. What was taken away from nature by the construction of the building was restored on the roofs. For the tree tenant, Hundertwasser has defined a list of points clearly supporting the benefits of integrating trees within building structures themselves: they create oxygen; they improve city climate and well being of dwellers (i.e. reducing the dry-humid and the cold-warm contrast); they act like ‘vacuum cleaners’ swallowing even the finest and poisonous dust; they swallow noise, reducing the echoes of the city noise and creating quietness; they create shelter and protect like curtains from the outside view; they give shadow in the summer but let sunlight through in the winter when the leaves have fallen. Living quality is improved by having a piece of nature of one’s own. The philosophy of the house also relies on humus and small sewage treatment plants with aquatic plants which replace water flush toilets to minimize water waste. Since its construction, this curvaceous and irregular-shaped house has been reviewed by the international press and visited by millions of people, becoming part of Austrias cultural heritage.