The extension's 12-meter high and 25-meter wide main facade shows a monumental artwork by Swiss sculptor and painter Josef Felix Müller. The 134 panels show life-size humans in a relief, created from molds that used the wood of a 130 year old sequoia tree. This tree had to be felled in order to make space for the new building.
Its permanent exhibition "100x Aarau" tells the history of Aarau through over 1000 objects and 100 stories.
In 2021, the Stadtmuseum Aarau hosted some of the exhibitions of the 4. Fotofestival Lenzburg inside the new extension building.