I am building a conservatory for myself and would welcome some advice on a concern I am having with the foundations. One side of the foundation has had to be excavated deeper (in places) than that of the other two sides due to the ground being a little softer. I have dug down to good firm ground now well in accordance with frost heave considerations. However the one side is approx 200mm in places deeper than all other sides. The only reason I haven't dug the whole strip to the same depth is that i've now reach what I think is sandstone and it is very hard to dig.
Would it be ok to fill with concrete now of do all sides have to be exaxtly the same depth ?