Make sure in advance you have a clay source. Don't take anyone's word. Test it, make cob bricks, dry them, load-test them, scratch for hardness. Foundation too low, discontinuous or lacking reinforcement. Footing trench not compacted.
Foundations are critical and nearly impossible to replace.
They should be numerous and well-marked, at each interior floor level, thresholds, and outside against the building. Foundations take longer than you think, especially rock walls. Beginning too late in the season. Cob needs time and dry weather to dry. Wet walls can be damaged by frost.