You can build off of concrete, but I would have to know more about the configuration of the concrete work, and what type of stem or "sill" wall you are considering to build, to give any clear guidance. Could you possibly post pictures.
Here are a few photos as requested. Basically the back and right side wall of the existing house will be kept and I was hoping to cob further out and extend the front all the way along the exiting concrete. The stem would will hopefully be made from existing rock from another building directly on top of the concrete.
I understand more now, but that has only generated more questions.
1. Is this in a desert environment?
2. Do you mind sharing the exact location?
3. How long have you been doing Cobb/Adobe work?
4. What is the circa date on the original building, it appears to be of vintage age?
5. The concrete appears to be crumbling in some locations, and unless you have extensive experience with concrete, I would recommend speaking with at least three concrete contractors of good reputation to look at the foundational elements. This structure appears vintage to me, that being said, the foundation may not me concrete at all, but stone with a thick concrete parging and in situ slab poured on top. If my assertion is correct you would be strongly advised to seek advice from a P.E. and/or a restoration specialist. If you have the skill sets, you can encapsulate the existing foundational elements in new dry laid stone work (properly done) and then continue to build as you see fit. I have reservation about destroying and/or modify vintage architecture without regard to the original craftspeople, and respect to their original work when ever possible; which includes doing new work in traditional systems as they would have.
Sorry for the late reply, I have been away for the past week.
Here are the answers to your questions.
1/2. It is not in a desert Environment, it is in the mountains in Bulgaria, in the Rose Valley.
3, I am quite new to cobbing but have done a couple of courses and feel confident to start building.
4, Not exactly sure of the date of the original building but only the back wall which is made of stone will be staying as the current side walls are made from adobe bricks which need renewing (cob walls will be used instead)
5, I will try to find out about the concrete foundations as I have a architect friend who helped me to purchase this and knows her stuff. I will let you know.
My main concern was whether you can actually build a stem wall directly onto the concrete as I have heard some people say the stem/cob walls all need to breathe and with concrete I may have problems with rising damp entering the stem wall as it will not have the usual rubble trench. Although after cobbing for the last 4 days with a group, the instructor did say I shouldn't have a problem doing it this way. What are your thoughts on this please?
Thanks for keeping me in the loop. I have enjoyed following along and will love to see your progress. I believe you can come directly off the concrete as well, as long as the concrete is of good construction and quality. Your architect should be able to aid in this. As for the capillary effect of concrete to wick water into the cobb, that is very possible, but can be subjugated by several means. Something a simple as a membrane between the cobb and the concrete could possibly work.
Have you learned about adding lime to the first course of of the sill cobb going into the stem wall?
Have you learned about lime plasters yet?
Are you going to have a stone course for the stem wall plaster in cobb?
How thick are the walls you plan on building? (I would recommend at least 600 mm for the sill/stem wall, tapering to 400 mm minimum for your region and climate.)
Are you doing any CAD or blueprints? I would strongly recommend doing so. It could aid folks like me assisting in the design process.
Will these structure you are building have any "bones" (i.e. structural elements) besides cobb?