I need to hook up a washing machine drain, It will be on the other side of the wall right behind the bathroom sink, can I tap into my sink drain?
If so, would I go under the floor?

I need to hook up a washing machine drain, It will be on the other side of the wall right behind the bathroom sink, can I tap into my sink drain?
If so, would I go under the floor?
We need a little more information. Connecting into the horzontal drain line or the vertical vent stack of the lavatory? If it's the vertical vent stack you could hold the sanitary tee close to the floor and install a 36" stand pipe. I've put up a rough drawing to help you. Good luck, Tom

Thank you! Yes, It will go in the vertical pipe. Now to locate the pipes?