I have a unfinished bathroom in the basement on a cement slab. The rough in plubming is in place with a about a 12x12 hole in the cement. How do I hook up the drain to the Ptrap. I do not have access from the side wall. Do I need to cut an access hole in the adjacent room?

What is the material already roughed in? PVC? Cast Iron? What size? Is there a pipe coming into the hole? Are you asking how to connect the trap to the tub or to the drain line? Where does the wall line come in conjunction with where you wish to place the trap. (Hint) The center of the trap raiser wants to be about 1 1/2" off the front wall line and about 15 1/2" off the side wall line. See the rough in spects for your tub. If the plumber roughed for a tub then you shouldn't have to open up a side wall. However you will have to have about a 14"X14" access hole in the wall directly behind the tub drain to connect the tub to the trap. I realize you will have more questions so ask away. Cheers, Tom

The material is PVC and it appears to be 1 1/2 to 2" I need to check. The pipe runs straight in to the hole from the wall where the drain side of the tub would be. so I am unclear whether this was roughed in for a tub or a shower. It seams like it would be difficult to attach a ptrap and get the center of the trap riser 1 1/2" from the drain wall. I the drain pipe is about 18" off the side wall.

You have options here. If you want to install a tub/shower you will  have to (1) build out the wall line to 1 1/2" center of the trap raiser and move the trap over 3 inches or break cement back to when you can position the trap or (2)  figure on building a tile shower stall to the existing rough in.  Your call.  Regards, Tom