I'm planning a basement bathroom where the plumbing was stubbed in during construction. The plumber installed a 2" pipe for a sink, 2" w/trap for a shower and 4" for a toilet. I would like to move the sink from the original location. I chiseled up the concrete and capped the sink line that was already stubbed in. I would like to move the sink to the other side of the room and tap directly into the main 4" stack.

Please see the attached picture which shows the old and new sink location. There are no vents installed yet. I'm guessing the plumber planned to vent the sink which I think would vent all three fixtures. Can I move the sink to the new location and vent it with an air admittance valve under the vanity? Would this take care of the venting requirements for all three fixtures? Is there any particular importance to sink (and vent) being upstream from the other fixtures in the original plan and downstream in the revised layout?

By the way, this new layout lets me install a 60" tub and 60" vanity where the old would have required a corner shower and 24" vanity.

Another thing to add is that the old sink location is probably at the end of the same line as the shower and toilet while the planned new sink location would be to go directly into the main stack.

You may move your lavatory but first you must know what's under the slab and how it's vented. In the old configuration the toilet drain line connected to the main. The lavatory drain line connected to the toilet drain and wet vents the toilet. The shower then connects to the lavatory drain line and is wet vented by it. By capping off the lavatory stub up you have capped off the groups vent.
To make this work you will have to uncap the lavatory stub up and use it as a dry vent for the shower and the toilet. In addition to that you will have to run a separate vent for the new lavatory. These two vents can be tied together and run out the roof, revented back to a roof vent in the attic or, if local codes permit, can be vented by one or two AAV's. Good luck, Tom

Thanks. That same thought crossed my mind last night when I remembered the old lav drain when up into a T, probably for a vent. Can I simply reconnect the line I capped into a vent and tap the new lav directly into the main stack with a AAV under the vanity?

Pictures below are what I'm looking at. The main stack is 7 feet from the toilet.

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Can I simply reconnect the line I capped into a vent and tap the new lav directly into the main stack with a AAV under the vanity

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Yes, If the vent you connect to is a dry vent,(no fixtures draining above it) and local codes allow a AAV. Cheers, Tom

Thanks for your help. I noticed that there is a 1.5" pipe that is Y'd off near the top of the main stack. This pipe goes along the floor joist into the unfinished room and then shoots straight up. I went up in the attic and can see that this pipe is connected to the main vent that goes through my roof. Like you suggested, I'll run a pipe straight up from the old lav location to this pipe to vent the tub and toilet and also tap into the pipe from my new lav to vent that.