Dormer extension plans elper.com although the terms are often used interchangeably, a "dormer" is a horizontal extension of an upper room that protrudes from a sloping roof, and a "dormer window" is a window located in that extension. house extension, loft conversion & building plans in. The height of the proposed dormer extension will exceed the height of the existing roof. the materials you plan to use in the construction of your loft conversion are not in keeping with the appearance of the existing house. the dormer loft conversion includes a balcony, veranda or raised platform.. Dormer loft extension this is as the above, but with the addition of dormer windows. this will increase the usable floorspace and can be used to add head height which gives you more options when it comes to placement of the stairs..
