Have to move washer and dryer stacked to this closet and I’m getting hung up on the plumbing drain details. Any advice would be appreciated. 

I have a washer box with hammer arrester ordered to run to the interior wall on the left, the dryer exhaust conveniently goes out the above grade exterior wall (right). no problem running electrical or supply lines. Closet is 38″ finish to finish width.

Ok so the drain. It seems that the main vent stack runs right through this closet (see photos left floor interior wall). In the basement the photos show main drain running down to sewer. The washer is also shown with original branch vent line of 2″ pvc. Which runs back to main stack at the sewer. Over the closet at the roof there’s an exhaust vent, but the pipe is buried behind walls, but I have to logically conclude it must be the main drain vent.

I’m weighting best practice here. I’m thinking AAV in a wall box, and tap into the main drain line…somewhere. The closet seems best, presuming i can frame around the pipes, there’s not much room to play with. Otherwise tap in below at the basement.

See I don’t really know plumbing that well as i.would like, is it common practice to cut into the main drain pipe like that for a new addition? I saw some articles about cutting and using couplers to add fittings, seems easy enough but I’m afraid of sewer gas if things go wrong.