Six inch walls.
You want to think about what happens to a 2x4 when a plumber and an electrician put holes in them to run their conduit and pipe.
A 2x4 is actually 1-1/2 inches by three and a half inches, and when they get finished cutting holes to run their pipes and conduits there is 3/4" on each side of those 2x4's left to hold your house up.
Ask for and demand six inch (2x6) walls along every plumbing wall. We will modify your plans to include such revisions, as well as make recomendations on things to look for in a well designed home.