Soil stack reduction

[JockoJr]

Can I go from a four inch drain pipe then reduce the soil stack to 3 inches(just under the subfloor below the bathroom floor) then go back to four inch in the attic - above the bathroom?

I need 1 inch of room for a 30 by 60 tub - the soil stack is pvc and I need to reduce it to 3 inches in the bathroom - to make room for the tub.

I'm restricted by a load bearing wall -main beam!

 

[Ben-gee]

What goes through this soil stack at that point?
If it’s only one toilet then it’s fine, if it serves multiple toilets then no.
If it’s just the vent pipe then it’s fine too.

This is practically speaking and I’m based in uk, I’m guessing you are USA where you’ll need to abide by your state’s plumbing code.