There are several different kinds of BFP. Some do have to be at the highest point in the system, but better ones (also more expensive) do not. You should check with your city to see what their requirements are. I use a Wilkins Double Check BFP that is installed in a plastic box below grade. I am required to have it certified each year by a licensed inspector. All BFP should be inspected yearly because they do wear out and require repairs occasionally, but some communities ignore the EPA rules.