I'm finishing my basement in a newer house and want to add a wet bar. There is a 2" PVC drain pipe coming from the 1st floor that is for the washing machine drain. It has a clean out plug on it also. Can I just add a wye or Santee for the bar drain and do I need an Air Admittance Valve for venting. I'm assuming the washing machine drain line is vented through the roof.
Do I have to build the wall with 2x6's?

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Yes tou may discharge into the 2" vent stack with a san tee and yes the bar sink will have to be vented. You may install a AAV if local codes approve, if not then a vent must be installed after the trap and run out the roof or revented back into a dry vent in the attic.
Good luck, Tom

Thanks so much Tom. What would be the best way to find out if local codes allow an AAV? And do I need to build this as a 2x6 wall? In other words, can I run the 2" drain line through a 2x4 wall? Does it have to be 2" or can it be 1 1/2"?
Thanks again, Dave

Make the san tee 2 X 1 1/2" an d the trap 1 1/2". Since you will have to cover either a vent pipe or a AAV a 2 X 6" wall should do it. Contact your local Building Department for information on installing a AAV. If they permit a AAV they may make you install a service panel to service it if necessary. It's not good practice to cover up a AAV so it can't be serviced. Good luck, Tom

Thanks a lot Tom. I'm trying to get ahold of someone at our local Public works to find out what the local code says about the AAV. I hate building 2x6 walls since it's hard to find good straight ones at Lowe's and HD but I'll live.

I just got the call from Public works and they said they don't really allow the AAV unless there is no other means of venting. They said I should be able to tie into the vent for the vanity sinkbut to just do it up in the floor joists area.

My code book says you can revent back into a dry vent, (such as your vanity) at least 6" over the floor rim of the vanity bowl. You may also run it it in the attic and revent back to any roof vent up there. Best of luck in your project, Tom