A dormer with side walls (1) and a barrel roof (2), With an obtuse angle (a) between the front side of the dormer and the ridge of the barrel roof The dormer comprises a window on its front side, with an arched top edge, fitted to the dormer roof arch (2), opened by the rotation of its sash (5) around its horizontal axis, where said sash is seated in hinges (6) fixed to the stiles (32) of the sash frame. The design of the window in the dormer is typical for roof windows.