We are mid-remodel. The house originally had 1 full bath on the 1st floor and 1 full bath on the 2nd. We are adding a second bath upstairs, and the 2 upstairs baths will share a wet wall ( the toilets are back to back against that wall).

The plumber says we may need to drop another drain line ... And I'm worried because I can't figure out WHERE that can be placed without tearing up the kitchen.

So, 2 questions...

1). Can 3 full baths share 1 cast iron drain (house was built in 1960's)?

2). If not, can a drain pipe run horizontally and cut through floor joists? I think that would require extra support for those floor joists somehow.

I do trust the GC but I'm also thinking that I want the best looking/functioning solution that minimizes the amount of demo to the first floor. And now I'm concerned about how that will be achieved.