I was very lucky when I bought my recent home. The previous homeowner had the original septic tank permit as well as several copies of the plan drawn by the septic designer.
The plan clearly shows the exact tank, the size and capacity in gallons of each chamber of the tank, and even the type and model number of the pump I need in my system. Unfortunately, my leach field is higher in elevation than my tank, so the effluent from the tank must be pumped about 30 feet uphill to the leach field.
These plans are like gold. If you have them, keep them in a safe place. The permit is also very valuable. Some states now require the seller of the home prove the septic system was authorized when it was installed. Without the original permit in hand, you may spend hundreds or thousands of dollars proving your system was installed correctly.
— Tim Carter