Re: Question about adding bathroom in basement. Drains
If this bathroom will make for 4 or more toilets in the house, then the main drain going out to the septic or public sewer should be at least 4". I'm not sure if there's any grace in the codes for adding to existing systems.
Now this can be a little confusing: it can be 3" so long as there are 3 or fewer toilets on that branch line. But wherever the total number of toilets feeding into the main line becomes 4, then it needs to bump up to 4". So if you have 3 toilets, you may have a 3" line down to where the fourth toilet connects in.
Sewer lines should NEVER shrink as they go downhill. That is, you can't have a 3" branch line connecting to a 2" branch line downstream.
The "Senior Member" designation under my name doesn't mean I know a lot, it just means I talk a lot.I've been a DIYer since I was 12 (thanks, Dad!). I have read several books on various home improvement topics. I do not have any current code books I can refer to. I was an apprentice plumber for two years.