Percolation test and setbacks are two of the more important compliance issues if you go new. Also, the existing may not be up to code for total length of the leach lines. They might wave some or all of these for refrubishment of existing. These would be the first things I would want to know. Also, if you build new, in a new spot, do you have to remove the old, revegetate the land or any thing else?
If the problem turns out to be smaller, at least you got an education from doing your due diligence.
