What's the requirement when a buried PEX supply crosses over a buried drain line?

I'm converting a garage into living space and am connecting the water supply, drains & electrical from the main house to this garage building. The city requires the garage to share everything with the house.

I'm still in the design phase but need to start plumbing and electrical in the next week.

My supply is PEX which will be buried in its own trench at 12" deep. A second trench for the drain/sewer will also be 12" deep. A third trench is the buried electrical conduit at 18" deep.

Is there anything special I need to do when any of these lines cross each other which they will have to at one point in the yard?

I think I could do one trench for both sewer & supply but I'd have to make it 24" deep and have a shelf at 12" which seems more difficult than just digging separate trenches.

Thanks for any ideas.

the PEX needs to be a minimum of 12" above the waste line

Plastic water lines typically have to be at least 18" deep to prevent damage. You will need a very experienced plumber to install the plumbing if the sewer trench is only 12" deep. Most need a 20"+ trench for the way they know how to do drainage piping.