Where to make the transition.

I am helping my son work on an old house he recently purchased. I have done plenty of plumbing, but this is the first time I have run into this.

In his powder room, he will be installing a pedestal sink (I hate them, but it is what his wife wants). The existing PVC drain line in the basement under this powder room is 1 1/2". All 1st floor plumbing was completely demo-ed before we got there. The pedestal sink, I assume, will require a 1 1/4" tailpiece and strainer, and an 1 1/4" trap. Where do I transition to 1 1/2"? I am thinking the best place would be at the trap adapter, but I would like confirmation.

Would it be better to transition after the tailpiece and use an 1 1/2" trap?

Thanks in advance.

they use 1 1/4 because the trap is smaller and easier to hide. there isnt much room to tighten the nuts. use the 1 1/4- 1 1/2 trap adapter on the wall takeoff like you suggested.

I have the nut of an 1 1/2 x 1 1/4 trap adapter sticking out of the wall. Presuming you are using a chrome trap then the escutcheon will cover the nut

Thanks to both of you that's what I thought also