I am planning on building a deck on a hill, the frost line is 42" and city requirements is 48". Do I need to dig even deeper since there will be two different levels of posts on this hill? The grade difference is about 3 feet from one post to the lower post and they won't be more than 6 feet apart.
you have to dig as deep as you do because you have to go below the frost line, to get to 'stable' ground the theroy is any thing above the frost line will move as the ground freezes and thaws.
your posts are all individual units. 48" if thats what it is, is what you need. the holes will follw the conture of the land. so if i understand your question the answer is no.