how old is your system? The city of Tulsa used to make it mandatory to have a seperatemetr for lawn irrragation. The other thing is , I don't think your irrigation backflow preventer is going to be by your water heater.That would reguire running a line from there under your foundation and stemwall.I would get a probe and start from your city water meter making circles.The main valve box shouldnt be more than a few inches deep and is about 1'x2' .And lastly, 1 valve per zone.I have seen them put the "main" in with like valva 1