We install water with sand if needed or native backfill. Sewer mains are laid in gravel and backfilled to above the pipe then native.

I really appreciate your quick and detailed response. I'll follow your advice. I also plan to tie our toilet into the existing septic tank while running grey water off (legal here). Is it okay to tie just the toilet in? Or should I also tie in the shower (or something) to help flush waste from the pipe to the septic?

NO GRAVEL I REPEAT STEP AWAY FROM THE GRAVEL !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! you want soft dirt around your water line, water lines will contract and expand putting a rock next to it is asking the rock to rub a hole in it over time my advice, do not use pex for your water service, use 3/4'' pvc instead or better yet. 1'' i like a large volume of water at my hose bibs. insulate the pipe, and use CHILDERS CP-11 VI-CRYL MASTIC COATING to paint the insulation to water proof it, available at a plumbing supply house use rubatex insulation with contact cement to stick it tigether