My footings go down 900mm, and sit on a 200mm strip foundation. I live in a shrinkable clay area and have minor cracks in my brickwork and strip foundations that are not moving and don't warrant underpinning. I want to build a small single-storey extension on one side of my house. I have been advised to build the foundations for the three other walls as trench fill, to the same depth as the main house, but not to tie them in. I understand the reasons for this in terms of not affecting the main house foundations. If I dig down and expose the main house foundation that I am extending from, is there a way to stabilize that foundation? It is sagging at one corner. I don't want to underpin, but had considered running a 400mm deep strip foundation alongside and above the current foundation, using steel pins in the lower footings courses to tie them and the old foundation into the new one. Does anyone who understands this area have any advice to give? Are there any methods of jointing the extension foundations to the main house foundations that still allow for some movement, in the same way as the wall starter bar I plan to use to join the new extension brickwork to the existing brickwork?