I know you're asking which is the right thing to do.
You need the code book for your Municipality. Yes, soil types and depths are of LOCAL importance. Technically, you'd need an inspection by your local municipality inspector.
If you don't get the Code, see if a local Plumber will help. I just can't give you the code for my Municipality. Next thing you know, you'll be asking us why your water bill is so high when you're not running the water. (It's because the pipe broke ...)