A behind-the-ear hearing aid unit comprising a housing comprising a hollow inner section defined by an enclosed wall surface made of a single inseparable unit; an electronic module; and an ear hook that is permanently or detachably attached to the housing; wherein the housing extends completely along a single curved axis running along the enclosed wall surface from a first end of the housing comprising the ear hook and a second end of the housing opposite to the ear hook, and the housing comprises an inlet opening at the second end, and the BTE hearing aid unit further comprises: a cover adapted to sealably close the hollow inner section at the second end, the cover comprises - a first set of connectors adapted to establish electrical connection with the electronic module with one end of the first set of connectors being accessible from outside the hollow inner section; and - a sliding guide rail configured to receive an attachable module comprising a second set of connectors and an interacting means adapted to run along the guide rail to establish a mechanical connection between the housing and the attachable module and an electrical connection between the one end of the first set of connectors and second set of connectors, and where the attachable module may include a battery compartment or a FM receiver. a housing comprising a hollow inner section defined by an enclosed wall surface made of a single inseparable unit; an electronic module; and an ear hook that is permanently or detachably attached to the housing; the housing extends completely along a single curved axis running along the enclosed wall surface from a first end of the housing comprising the ear hook and a second end of the housing opposite to the ear hook, and the housing comprises an inlet opening at the second end, and the BTE hearing aid unit further comprises: a cover adapted to sealably close the hollow inner section at the second end, the cover comprises - a first set of connectors adapted to establish electrical connection with the electronic module with one end of the first set of connectors being accessible from outside the hollow inner section; and - a sliding guide rail configured to receive an attachable module comprising a second set of connectors and an interacting means adapted to run along the guide rail to establish a mechanical connection between the housing and the attachable module and an electrical connection between the one end of the first set of connectors and second set of connectors, and where the attachable module may include a battery compartment or a FM receiver. - a first set of connectors adapted to establish electrical connection with the electronic module with one end of the first set of connectors being accessible from outside the hollow inner section; and - a sliding guide rail configured to receive an attachable module comprising a second set of connectors and an interacting means adapted to run along the guide rail to establish a mechanical connection between the housing and the attachable module and an electrical connection between the one end of the first set of connectors and second set of connectors, and where the attachable module may include a battery compartment or a FM receiver.