Adding a room onto an existing home is probably the most cost effective way to increase a buildings usable interior space. In this article, an addition means what a colleague calls a three sided addition. This phrase intends to avoid confusion with other sorts of home additions such as raising a building to create a new ground level space, or raising the roof to create a story between a ground level space and a roof area. The three sided addition means that the new and existing building will share an interior wall.
The first consideration when planning an addition is headroom: the height of a ceiling relative to human proportions. Most building codes stipulate minimum ceiling heights, but, as most people prefer ceilings that are at least eight feet (2.5 m) high, a well-designed space will probably meet or exceed these. Ensuring adequate headroom is probably the most challenging aspect of addition design, and is the main reason to start planning an addition from the roof down.
Begin your design thinking by trying to envision what you consider an ideal ceiling height for your addition when finished. As mentioned, most prefer a minimum eight feet, but a few inches less than this will still work in a pinch. It is important to start here, because your new ceiling will likely be hanging from the roof framing that will, in turn, attach to the existing building. If this framing attaches to an existing building too low, your ceiling will be too low. Lets look at a couple standard roof frame techniques to help clarify.