I've been building a house for about 8 months and I'm about ready to tie the sewer into the existing septic system. About 5 years ago I built a shop with bathroom in it, with plans to build the house so the septic is was permitted and installed with the shop.
I attached the pic to show the setup, now the question is how is the best way to tie it in? I could put a new hole in the first tank, or tie it into the existing line. If it's tied into the existing line should it make a U turn so it shoots toward the tank into the old line?
You should be getting a permit and inspection to tie the house into the existing septic system. The system may have been designed just for the shop, or it may have been designed with a house addition in mind. The difference would likely be in the size of the drainfield. The local Health Department septic permitting technician could advise you best. There should be a record of the original system.
Is there a knockout hole in the house side of the tank nearest the shop? I would dig around the upper part of it to see. That would be the best tie-in.
If not, I would run the house line to the line from the shop just before the first tank.
Use a street T for the connection aimed downstream to the first tank and don't forget to put a clean-out on the house line.