I plan on adding an addituional bathroom back to back with an extisting one. Both would be first floor installs with easy basement access. The toilets would be 7 to 8 feet apart. Is it permissible to use the same drain with exiting vent?

You ask about adding a toilet but you indicate adding a bathroom. If you're adding a bathroom will it be a 1/2 or full bath? Are you adding a bathromm or just a toilet? Please be a little more specific. ( I'm a literal kind of guy) Regards, tom

It will be a full bath(shower,toilet,sink).The two bathrooms will be back to back.

I plan on adding an addituional bathroom back to back with an extisting one. Both would be first floor installs with easy basement access. The toilets would be 7 to 8 feet apart. Is it permissable to use the same drain with exiting vent?
Yes, Configure the lavatories back to back so the original vent will service both bathrooms.
Let me show you a typical rough in. 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.. Good luck, Tom

Thank you, Tom. I really appreciate your advice.

Take care,
John