My in-laws are building a little place up on the side field at my farm. I am thinking about running their washing machine water out to the side and not in to my septic tank. Would this just have a p-trap and then run the discharge tube into some gravel?  If I wanted to save this water, what would I need to do about the soap. Bathroom sink could go this route as well?
There is no code out here. I have never had any septic issues and have a full sized system but I don't want to stress it with the additional home.

Run everything but the shitter out to its own leach field.

This works well.

I was going to do this at our cabin up north but didn't after a plumber friend advised you really want some clean water going down to help scour the line like from a shower.

I decided not to chance it and so all bath fixtures go to one line and kitchen goes to a gray.

I have a friend who had everything but the shitter running into a drain which ended on the edge of a field. Worked fine for many years, no septic issues and the grass grew like crazy at the end of the gray water drain.

Kitchen sink and shitters to the septic. Bathroom sinks, washer, and bathtubs to a graywater leach field.

In addition to reducing the total water running through the septic, bleach in any amounts is not great for your septic.

...but, add a settling basin\filter for the graywater.  You will be amazed at how much hair, clothing fibers, and sand end up in there.  Without it, the leach field for the graywater can clog over time.

I've seen that done all over Tx in rural areas; alot of folks run long (20-30') 2' PVC lines & move it around
weekly to water trees & yard; make sure soap is biodegradable (most are) & u have no nosy
neighbors that would bitch.

technically, per TCEQ you cannot discharge grey water onto the ground (don't tell me about golf courses using it),
but if nobody invites the man into your life, yur GTG; I would do this if building my own place