Does the garage go on the left or the right of the black line? The front of the house isn't completely obvious there.
The OP can clarify but I think we are looking at a side shot of his house. On the right, you can see what I believe is his back patio. To the left of the black line, I believe is one of the sides of his existing garage. It appears he has a man door from the garage to the side yard, with a/c units nearby.
My interpretation is the black line would be the back wall of his future garage; therefore, making the front entry to the left.
Assuming all this correct, OP, how do you plan on addressing the a/c units?
I haven't used a traditional septic system; however, with (2) previous homes I had the "new" style that has a 3-stage tank, and then shoots out to (3) strategically placed sprinkler heads. So you have a sprinkler system for your backyard.
Seems weird, but supposedly by the time the water hits the sprinkler heads it's supposed to be clean enough to drink. I never tested the theory but can vouch the water is clear when it comes out. Also, at the first house, we had company over and a bunch of kids in the backyard when the sprinklers kicked on. It was summer time and the kids went crazy playing in them. None of them stunk or had any craziness go on. However, after they were alerted what type of sprinklers they were playing in, it was a huge buzz kill for them. For the parents, it was a huge laugh.
I think each sprinkler head had a 25' radius and overlapped a bit.
What I do know about septic systems is this "newer" style like I had is used on newer homes, and also homes where the land won't perk right for a traditional system. It wouldn't be cheap, but theoretically you could probably convert and run your sprinkler lines so it works with your garage plans.