Goebel Residence - Palo Alto, CA
This 180 square foot office addition required a high level of creative thinking in the interface details of the walls and roof. Additions are notoriously more complicated than new construction, in that the existing conditions must be addressed in great detail and with consummate care in order to maintain the design intent of the original structure, and to ensure water-tightness at geometrically complex intersections of exisitng and new materials.
In addition to an existing residence, there are a multitude of conditions that are not specifically documented in the construction drawings, because they are revealed during construction. It would be impurdent to cut up an existing house prior to securing financing/building permits to determine all of the facts of the case. This is a primary reason so many residential projects go over budget. My experience with projects like the Goebel residence informs my estimate of contingencies required for a realistic project budget.