Hi we are putting in a basement bathroom, and I was wondering about the venting. THe basement bathroom will be directly below the upstairs bathroom . Do I just hook my basement venting into the line that runs from the upstairs or do I rum a whole new pipe through the roof? And where should I install venting in the line ( before the toilet after the toilet or the sink or what?) Any advice would be appreciated towards the plans Thanks

Most bathroom groups are roughed in like this.
Toilet connects to sewer main or the stack vent. Lavatory connects to toilet drain and runs a vent off the top the stubout tee out the roof or revents back into a dry vent in the attic.. The toilet wet vents through the lavatory vent and the tub/shower connects to the lavatory drain and is wet vented by it. This is a normal rough in and is acceptable both by local and state codes and also The Standard Plumbing Code Book in 90 percent of the country. Check your local codes.to make sure you're not in the excluded 10 percent.
The vent off the lavatory may be run out the roof or revented back into a dry vent in the attic or if you're reventing back into a fixtures dry vent you must make your connection at least 6 inches over that fixtures flood rim.
Good luck, Tom