Dormers can add greatly to the exterior aesthetics of a house, as well as provide more space inside. But because of their prominence, poorly designed dormers can mar an otherwise pleasing facade.

To design dormers the right way, follow these rules:

Design dormers in a style appropriate to the architectural style of the house. Shed dormers, for example, are simple and well-suited to a cottage or another unadorned style. A gable end dormer, on the other hand, is more formal and better suited to more traditional styles, like colonial or Georgian.

Keep the scale and size proportionate to the size of the house. Don't build dormers so large that they tower over the house and seem ready to crash through the rafters. A way to quickly test for oversized dormers is to see whether the dormer's ridge line competes for prominence with the main ridge line.

Make sure the window covers enough on the face of the dormer. The window should cover at least 75% of the dormer's face. Trim should occupy most of the leftover surface area. Too much siding or stucco on the dormer face makes the dormer appear heavier than it is, creating the impression, even if it's sized correctly, that the dormer is too big for the house. --Brent Hull is a millwork consultant and author of Historic Millwork. He can be reached at