The conversion of the older plenary building, which was completed in 2008, was designed to meet the needs of over 100 MPs and their staff, a large number of media representatives and around 50,000 visitors a year. On the other hand, the urban planning aspects of the old town with its springs, the nearby Bäckerbrunnen fountain and the surrounding streets were also taken into account. The new plenary building is located on the site of the former riding hall of the Wiesbaden City Palace, to whose original dimensions it was restored. This created new street space in Grabenstrasse.

The plenary building was largely harmoniously integrated into the existing ensemble of the historic city palace and the surrounding old town buildings. The Prinzengässchen, which once ended as a dead end from Marktstraße at an insurmountable gate, now opens up into a new public square. The natural stone-clad façade of the plenary building allows daylight to enter only through "points of light" and gives the building a special appearance at night. On the level below, on the other hand, the large windows allow a view from Grabenstrasse across the inner courtyards of the Hessian State Parliament to Schlossplatz. The connection of the plenary hall to the historic city palace was realized by a transition at the level of the historic music hall. The seating is arranged in a circle based on the image of the "round table" and in the sense of a "dialogical space". The visitors' gallery rises above the central location of the political events. Other interior spaces of the complex are connected via galleries and air spaces with a variety of visual connections between the individual components and levels. A visitor center informs citizens about Hessian state politics.