I had one bathroom, we recently added a 2nd bathroom. The 2nd bathroom comes before the original one. I am not sure where to vent at. We have a terrible septic smell in the original bathroom.
Each individual plumbing fixture is assigned a value called a "fixture unit" The units are then totaled and this determines the size of the vent stack,etc;etc. Some things you can check yourself are: Make sure each individual fixture has a trap. All plumbing fixtures are required to be trapped. Make sure you have a good tight wax seal on your toilets, sewer gas can escape if the seal is lost. Check the toilets to make sure the bolts are tight and they don't rock. If your not sure about an additional vent stack and / or code compliance call in your plumber. Sewer gas will make you sick.