OK, he is standing right here and says that states, townships, counties, cities and even inspectors can and do differ in what they require or will allow. Sometimes it makes it really hard for them to do their work. In Michigan, the specs were changed a few years back and some of the older inspectors who know Pete would let him plumb and hardly ever red tag his work. But the younger ones who didn't know him, were more apt to make him use the newer techniques. Or some of the inspectors were power hungry but he knew which ones would give him a hassle.
He says if you want to spend extra money, run it above. It is also easier to fix things in the future when the pipes are run overhead. But also, it may be a more major tear up to run it through the walls and and in the attic. When plumbing is run through the ground, there is the possibility of things settling eventually and something springing a leak. That seldom happens overhead. He said, not knowing exactly what you are experiencing, that his gut would be to tear up the floor and fix the leak and live with what if.
Remember your fixit guy is in it for the money, maybe not your best interest. Did he come well recommended?
How old is your plumbing? How did you discover the leak? Do you have an idea where the leak is?