The building has an unusual, bulbous shape, purportedly intended to reduce its surface area and thus improve energy efficiency, although the excess energy consumption caused by the exclusive use of glass (in a double facade) overwhelms the benefit of shape. Despite claiming the building "demonstrates the potential for a sustainable, virtually non-polluting public building",[7] energy use measurements have shown this building to be fairly inefficient in terms of energy use (375 kWh/m2/yr), with a 2012 Display Energy Performance Certificate rating of "D".[8] It has been compared variously to Darth Vader's helmet, a misshapen egg, a woodlouse and a motorcycle helmet. Former mayor Ken Livingstone referred to it as a "glass testicle",[9][10] while his successor, Boris Johnson, has referred to it as "The Glass Gonad"[11] and more politely as "The Onion". Its designers reportedly saw the building as a giant sphere hanging over the Thames, but opted for a more conventionally rooted building instead. It has no front or back in conventional terms but derives its shape from a modified sphere.
A 500-metre (1,640 ft) helical walkway, reminiscent of that in New York's Solomon R. Guggenheim Museum, ascends the full height of the building. At the top of the ten-storey building is an exhibition and meeting space called "London's Living Room", with an open viewing deck which is occasionally open to the public. The walkway provides views of the interior of the building, and is intended to symbolise transparency; a similar device was used by Foster in his design for the rebuilt Reichstag (parliament) in Germany. In 2006 it was announced that solar photovoltaic cells would be fitted to the building by the London Climate Change Agency.
The council chamber is located at the bottom of the helical stairway. The seats and desks for Assembly Members are arranged in a circular form with no clearly defined "head", podium, or chair where a speaker, council chairperson, or mayor might be seated. Raised tiers of seats for visitors or observers is located to one side.