[Cob] building on ledge

[Shody Ryon]

I have not been schooled in this or anything to do with building. Depending on some factors I would say, maybe. If you have no idea but it is close to the edge, the building would not be problem, if the ledge itself fell, that would be a problem.
I am not sure about cob specifically, but in general, building on rock is desirable because it doesn't settle. A problem might be if a great deal of water flows on the rock, from rain, in which case a rubble foundation might be needed to break the capillary action and a water diversion system might be needed, I am really guessing, I don't know if that would be needed or how to do it.
If there are too many unknown variables, a really strong but small concrete and steel perimeter foundation might make things more certain, one that would keep the house stable and supported if a part of rock broke away from underneath. If that is a likely possibility, it is probably not a really fantastic sight, in that regard.
you can try to speak with a local surveyor, if they are not real busy you could ask their opinion about building there in general. I wouldn't think the fact that cob is the material of choice would make a lot of difference  as to if the site is suitable to build, so I would leave that out of the discussion at first, because that will become the focal point of the discussion for no good reason. It even tricked me, who should know better.
Hugs,
Shody