It’s not as simple as just pulling the permit or adding the value right before you sell. The jurisdiction can make you open all the walls to verify sworn was done to code, impose fines, and if codes changed they can make you update the work to the new codes. Any good realtor will pull a history of work on a property so it’s not like you can sell your 2 bath house with 3 bathrooms unless it’s documented.

If you can’t afford to do it right you can’t afford to do it at all. That toilet will back up it will flood and then you will have to demo out the basement ceiling to repair the damage. One way or another the work will have to be done.

Find your 3” or 4” line and do it right, pull a permit and have it added to the value of your house. It will make your home more valuable in the long run.