Per R101.4 of the Oregon Residential Specialty Code, the intent of the code is to ensure the the safegaurding of public welfare through
affordability,
structural strength
means of egress
stability (seismic activity)
sanitation
Light and ventilation
energy conservation
and protection of life and property from fire, flood, and other hazards associated with a built environment.
Also R104.11 says that the provisions of the code are not to prevent the installation or use of alternative materials or to prohibit the use of any techniques or methods that are not prescriptive to the text. However the building official may require that you have tests done to prove that your alternative method, technique, or material is at least as good if not better than what is already in the code at ensuring those things listed above in the intent as listed in R101.4.
So to get a cob house permitted, you need to either have tests done to confirm that cob meets the intent of the code in all the areas listed in R101.4, OR you can get an engineer to stamp their approval of your design saying that from an engineering standpoint, your design is safe and meets the intent of R101.4. I think most people go with the engineer because it's probably more affordable and quicker to get the design from one person rather than getting tests done on the material for all the areas listed in R101.4.
There is a form in at least some of the jurisdictions, Clackamas county for sure, to apply for the use of an alternative material or technique.
Hope that helps.
