The 120-strong staff of the WNF, the Dutch wing of the World Wildlife Fund, were housed in a converted office building from 1954 once belonging to a research institute for animal husbandry. The WNF wanted a sustainable, low-energy building in harmony with the surrounding nature. The existing building was stripped and given a new wood-framed curtain wall. The ceilings now have a five-centimetre-thick clay finish with a system of waterpipes for storing the released heat. Solar panels provide electricity and warm the water. In the centre of the building a new spherical element has been added that ranges the main entrance, the lobby and a call centre round a central well. An aerial walkway leads to a meeting room housed in an existing annexe. The concrete structure has a ceramic tile cladding.