I am in the process of building a second story over my garage. It was discovered by digging down and a small channel under the foundation, that the existing foundation width was 12". It was a dissapointing moment since I knew that the code required 16" for a two story structure.
It was then necessary to dig a 2' trench beside the existing foundation and also 6" under the existing foundation. This was filled with concrete and extended the existing foundation.
The project went surprising well and has been one of the easy parts of the project.