I'm adding a small 1/2 bath to my basement. See the pic below. The 4" main drain line is exposed and intersects the bathroom diagonally. I happened to luck out that the toilet rough in lines up exactly on the center of the existing line - 12" out on the framing square in the photo.
I'm not exactly sure how to vent the toilet. I will be installing a sanitary tee to connect the lav along the line of the level which will also serve to vent the lav. Will this vent be sufficient to take care of the venting needs for the toilet?
Also - there is a chance I'll be adding a bathroom upstairs in the near future almost directly above this bathroom. Instead of coming off the main line with a 2" for the lav, I considered bringing a 3" line vertical to serve as a future drain stack. Right now, I would cap it with an AAV. Here's my issue - if I use this 3" as a drain in the future, the vent would become a wet vent to the basement bathroom. Would this be legal or would I need a separate vent line for my basement bathroom?
Thanks in advance for the help.