Definitely worth talking to your local guy to see what is required. My good friend built a house in the country, knew better but let the GC spec everything. He got concerned when he went out one day and they were getting massive excavation done for the septic. Some words, a septic engineer and awhile later, his leach field went to half of what the GC was going to put in. I learned from that, did the calcs myself based on the law, had the local inspector check them and was allowed to do my own install. $200 of excavation, parts from the local wholesale plumbing supply and a tank from the local spetic place which they drop in the hole and I was set. I don't know if it is law where you are, but it is a good idea - here in NY, you have to be able to demonstrate a viable plan for "doubling" your leach field, so that if your system dies out or has problems, you can add to and or replace it. Food for thought.