I'm hoping to add a second bath to the second floor a 101 year old house remodel and am having trouble finding a suitable place to put the soil stack. Can I place it outside the dwelling down an exterior wall? If yes, do I have to build a chase around it (I live near Seattle)? Also, does the full 4" stack need to extend above the roof line or can I vent upwards above the wye with a smaller diameter vent line?
You would need to check with the local inspector, I think you can in King County.
If you have other vents in the home, you could get by with a 2" vent for that bathroom set through the roof.
A 2" is pretty easy to flash. The last time I did that for a 2" vent run from the basement through the overhang in Seattle, I roped myself off while on the roof.
I used my harness for mountain climbing.