2nd Floor Bathroom Covert Single to Double Sink

[Dave K.]

Hi everyone. I'm rehabbing our 2nd floor bathroom which currently has a 72" vanity area with a sitting area and a single sink. When I took that out and opened up the drywall the trap arm for the current sink is 1 1/2" into a 3" stack. Where the second sink will go there is existing 1 1/2' pvc that is coming up from the basement off of 3" and running up through the attic. If i need to tap into the 3" for the second sink/trap I know what I need to do. But my question is could the existing 1 1/2" pvc have been installed when they built the house to anticipate a second sink/trap arm at some point in the future? Is there an easy way to determine if if I can use this existing 1 1/2" pvc? Would save a lot of extra work if so. Thanks.