I am a laid off apprentice plumber and I cannot remember the distance code for a toilet. Is it 18" on center from side walls and 10" from the back? I am going to be doing an underground for my Mother in law, she is putting an addition on. She is going to have 1 toilet, 1 stand up shower, 1 bathroom sink and 1 kitchenette sink. Should I run the 3" for the toilet and branch off with a 2" to the 2 sinks and shower? Where along that should I put in a vent? Both sinks will be on the outside walls. The main 3" will then tap into the 4" before the septic tank. Does this sound right?