An extension device for a window or a door having a frame and a sash movable in relation thereto having a sash frame, which comprises at least the following features: a. one or more connectable connecting rods (9, 10) which can be articulated with the sash and the frame at least at one point, b. an energy accumulator, which comprises one or more springs and a pull and push rod, which is attached directly at a point to the connecting rod of the extension device, c. wherein one or more of the springs are provided to assist the movement of the connecting rod of the extension device at the point at least in a first movement direction by way of a force exertion on the push and pull rod and/or to damp the movement of the connecting rod of the extension device at the point by way of a force exertion on the push and pull rod in at least one second movement direction (-X) opposite to the first movement direction, d. wherein the extension device is designed as an extension scissors hinge, having a main connecting rod and an auxiliary connecting rod, wherein the connecting rod on which the pull and push rod engages is the main connecting rod, e. wherein the energy accumulator comprises at least one housing, which has an elongated shape and in which a carriage is displaceably guided, on which the pull and push rod engages, characterized in that f. the pull and push rod is attached at a location on the main connecting rod, which is located spaced apart from the location at which the main connecting rod can be linked to the sash, g. the housing of the energy accumulator is arranged parallel to the main extension direction of a sash frame spar on said spar in the installed state, and h. an end face of the housing comprises an opening, out of which the pull and push rod protrudes in a longitudinally-displaceable and pivotable manner, wherein the end of the pull and push rod protruding outward out of the housing is arranged on the main connecting rod of the extension device, and i. the location of the linking of the pull and push rod is located offset from the location of the linking of the main connecting rod on the sash frame in such a manner that the location describes an arc around the location during the opening and closing of the sash. a. one or more connectable connecting rods (9, 10) which can be articulated with the sash and the frame at least at one point, b. an energy accumulator, which comprises one or more springs and a pull and push rod, which is attached directly at a point to the connecting rod of the extension device, c. wherein one or more of the springs are provided to assist the movement of the connecting rod of the extension device at the point at least in a first movement direction by way of a force exertion on the push and pull rod and/or to damp the movement of the connecting rod of the extension device at the point by way of a force exertion on the push and pull rod in at least one second movement direction (-X) opposite to the first movement direction, d. wherein the extension device is designed as an extension scissors hinge, having a main connecting rod and an auxiliary connecting rod, wherein the connecting rod on which the pull and push rod engages is the main connecting rod, e. wherein the energy accumulator comprises at least one housing, which has an elongated shape and in which a carriage is displaceably guided, on which the pull and push rod engages, characterized in that f. the pull and push rod is attached at a location on the main connecting rod, which is located spaced apart from the location at which the main connecting rod can be linked to the sash, g. the housing of the energy accumulator is arranged parallel to the main extension direction of a sash frame spar on said spar in the installed state, and h. an end face of the housing comprises an opening, out of which the pull and push rod protrudes in a longitudinally-displaceable and pivotable manner, wherein the end of the pull and push rod protruding outward out of the housing is arranged on the main connecting rod of the extension device, and i. the location of the linking of the pull and push rod is located offset from the location of the linking of the main connecting rod on the sash frame in such a manner that the location describes an arc around the location during the opening and closing of the sash.