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Have received planning to build an additional bedroom and bathroom on top of existing kitchen/flat roof. I have dug down to expose the existing foundation, and the concrete foundation is roughly about 7 - 8 inch thick, the ground below this is mainly bedrock and large stone.
The building inspector came today and asked me to expose more of the foundation so he could inspect further, I assured him that the thickness of the concrete was as above but he wants to return once i have dug further.
The extension above is only 3m x 6m with the (wall-plate) wall only about 6 courses high. I don't want the building inspector to return and inform me that i have to underpin the the foundation in order to build above, surely 7 - 8 inch of concrete on the mainly bedrock/large stone is ample to proceed.
If i have to underpin would timber-frame be an option or is there other ways please reply !

Just please do what the inspector wants. The rules are not there to make your life difficult but to make sure your home is safe for you and future occupants. Whether or not it sounds good to a brickie or not does not matter 1 iota as this is ridiculous advice. Do what the regulations require as you are legally bound to do once you have established that Building Regulations are required for your project.

The inspector simply wants to establish that the thickness of the foundations is fairly uniform over a length of the foundation as, in many many cases, when poor builders reach an area of rock or stone they simply dig over it instead of through it. If the foundations are thicker, or thinner, either side of the area you have exposed there is danger of cracking when the extra load is placed on it. Builders who know what they are doing will understand exactly why this additional exposure is required so do not use or take advice from anyone who thinks they know better than the authorities who are in place to keep you safe.