foundation depth

[bernardgreen]

you can see walls that are out of plumb, yet have stood for hundreds of years.

the walls have moved from vertical as built as a result of the ground around the foundations allowing the foundations to move over hundreds of years but those wall were heavily over engineered and over sized.

When we built a house on 22 pad foundations each hole was 2 foot by 2 foot by 4 or 5 foot deep. The Building Control Officer and the architect required that the bottoms of these holes were flat and level. Otherwise there would be a possibility of the base of the concrete moving sideways over time.

 

[John D v2.0]

This feels like a repeat of the birdsmouths in rafters debate from the roofing forum!
Just remember that forces are perpendicular to the plane of contact.

 

[chappers]

I would be inclined to just bite the bullet and dig to 1m at the lowest point with a level bottom back to the high point. But it can be said that you could take it to 1m at the mid point of the run.
But as Woody alludes to it's not just about depth but about ground conditions. You might find you go to 1m at the lowest point and are still on unsuitable ground