An extension device for a bone anchor is provided, wherein the bone anchor comprisesan anchoring section (100) for anchoring in a bone anda receiving part (200) connected to the anchoring section, the receiving part comprising a central axis (C) and a channel (204) for receiving a rod, wherein sidewalls of the channel form two free legs (205a, 205b),the extension device comprisinga first sleeve (1) with a first sleeve axis (c) that is coaxial to the central axis (C), wherein the first sleeve (1) is configured to be coupled to the receiving part (200) and decoupled from the receiving part (200)a second sleeve (2) with a second sleeve axis (c) coaxial to the central axis (C), wherein the second sleeve (2) is positioned within the first sleeve (1) and positionable relative to the first sleeve along the central axis (C) and wherein the second sleeve (2) is configured to be coupled to the receiving part (200) and decoupled from the receiving part (200)a locking member (35) that in a first configuration inhibits decoupling of one of the first sleeve (1) or the second sleeve (2) from the receiving part (200) when the other one of the first sleeve (1) or the second sleeve (2) is still coupled to the receiving part.
