The Rogers Stirk Harbour + Partners says this about the concept of the National Assembly building, "RRP employed the idea of openness and transparency as the driving factor in the design. The building was not to be an insular, closed edifice. Rather it would be a transparent envelope, looking outwards to Cardiff Bay and beyond, making visible the inner workings of the Assembly and encouraging public participation in the democratic process."

Regarding the brief they add, "The specifics of the brief included the stipulation that the building be an exemplar for access, that sustainable strategies and renewable energy systems be implemented throughout, that the building have a minimum 100 year life span, and that, wherever possible, Welsh materials be used. Other elements included a 610 square metre (6,566 sq ft) debating chamber for 60 to 80 members, three committee rooms, offices, a media briefing room, tea room, members’ lounge, public galleries, and a main hall to act as reception, orientation and exhibition space. "

Without question the most startling feature of the inside of the building is the funnel that rises above the debating chamber.   The inside of the funnel is clad with aluminium tubes arranged horizontally to follow the profile of the structure.  However, it is the outside of the funnel, that you see rising like a giant mushroom to the undulating ceiling, that catches the visitor's eye.  It is clad in Western Red Cedar from Canada selected for its beautiful appearance and its flexibility.  BCL Timber Projects were responsible for this ceiling using 60,000 linear metres of cedar.