I have an old concrete garage, circa 1950's which I'm using as a workshop. It is very dilapidated and is a matter of time before if collapses on my head. My plan is to demolish and then build a new workshop using concrete block. I have no idea of the condition or depth of the foundations and in any case I will be building it laterally to the left. Nearly all the houses on my road have some kind of detached building in place of the garage so I don't think I need planning permission, the only no no is on the actual height. My question is simple, for a concrete block structure with walls no higher than 2.5mtr, what is the recommended depth to dig down for footings ?