The Signum used the long wheelbase version of the GM Epsilon platform also used by the Opel Vectra Caravan.

General Motors, Opel's then parent, executed a similar concept in North America of a large hatchback with the related 2004 Chevrolet Malibu Maxx, also derived from the Epsilon platform.

There is also a very narrow central section, which includes a folding armrest and can also serve as a third seat (the Signum is fitted with three rear headrests and safety belts).

The seats can also be folded down individually to increase the cargo space, a system that Opel calls Flexspace.

The Signum shares many body panels (including the complete front part of the body) and interior elements (in particular the complete dashboard and front seats) with the Vectra, but the tailgate and rear fender design is unique to the Signum.