An extension device includes a patient support surface and an extension beam (16) that can be connected via an interface (14) to the patient support surface and that has a holder for a feed rod arrangement. The patient support surface has at least two longitudinal beams (10) parallel to each other that carry a pelvic support plate. The interface (14) includes a coupling part (36) on the support surface side and a coupling part (38) on the beam side rigidly connected to the extension beam (16). The coupling part (36) on the support surface side on one of the longitudinal beams (10) is supported such that it can pivot about an axis perpendicular to the pelvic support plate, and whereby the coupling part (36) on the support surface side can be locked by a locking device, which may be actuated by remote actuation, against a rotation about its pivot axis relative to the longitudinal beam (10).