A pre-formed glazing unit for installation in an opening defined in a structure such as a shed, greenhouse or other temporary building comprises a face panel having a periphery enclosing the face panel and a mounting flange for attachment to an inward side of the structure around a peripheral edge of the opening in use, the mounting flange being connected to the periphery of the face panel by a connecting wall. The connecting wall is integrally formed with the face panel and the mounting flange and extends away from the mounting flange to space the face panel from the mounting flange in an outward direction. The face panel and/or a top panel of the connecting wall may be inclined and/or a bottom panel of the connecting wall may include a drip-inducing formation to direct water away from the structure in use. A double-glazed variant is also disclosed.