Im planning on adding on a bathroom. Just a toilet and a sink.My question is on the drain for the toilet,It's about a ten foot run over to the main line witch is vented straight to the roof.I can connect to it through an existing cleanout right under the house.But my question is about how to vent it.Do I need to.Upstream or down?? Can I vent it through an existing vent for the tub and sink in the bathroom right next to it?? :confused:

You connect the toilet to the main. You connect the lavatory to the toilet branch and run a vent off the lavatory where it will not only vent the lavatory but also wet vent the toilet. This vent will tie back for a revent to the existing lavatory vent in the next bathroom. Good luck, Tom