I'm doing a bathroom remodel in which I plan to swap the locations of the toilet and the shower. All of my DWV is ABS and it is located in an accessible crawl space below my single story house. I believe the swap is going to be pretty straightforward but wanted to make sure I'm not missing something.

In this post I will ask about the drain for the new shower. Here is a photo of the existing toilet and lavatory drain. The toilet is 3" on the right, then meets the 2" vent which is also the lavatory drain. Then it flows west to my septic tank.

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Now I want to remove the toilet and put a shower drain in its place. Assuming the maximum dimension to the vent is not exceeded, is there anything wrong with doing it this way? I'm hoping I can still use the same shared vent (shared with the lavatory) for venting my new shower and not have to vent it separately.

What's the cleanest way to tie in the new shower drain? It will be 2" ABS. What fitting(s) would you recommend to transition the new 2" shower drain into that existing 3" line (where the toilet currently connects)? The new shower drain will be in the same N-S plane as the toilet drain, but about 18" farther south (away from concrete foundation) than the existing toilet drain.

Thank you for any input/advice.

Heidi