Plumbers?

Any plumbers on this site?
We built our house last year. It is raised with the garages underneath. When it was built there was an extra 3 or 4" waste line plumbed into the garage. It was not used at the time, and I want to add a toilet in the garage.
Is it as simple as running a cold water line and running the proper PVC to connect to the waste line? Or does this need to be vented?

Thanks in advance!