I am trying to plan out a new bathroom and the sewage inlet is directly below the bathroom (inside the house) but I ideally I want the soil stack to go outside the house.
Is it possible for the soil stack to be outside and return inside at ground level to join the sewage inlet?
External soil pipes are frowned upon. Various reasons one of which is that in cold weather they can become constricted by a build up of ice inside the pipe.
If the local Building Control allow the external then think carefully about how you will deal with a blockage in the S bend.
Steep slopes as in your second option lead to the flushing water and the solid matter separating leaving the solid matter to dry and harden in the pipe.
The soil waste ideally should go directly vertically down, this allows for solids to pass through the middle of the pipe and flushing water on the outer internal.
Tocky - you're getting so much mixed info I'm going to keep quiet and suggest you ask your local building control. Because I'd say none of the above apart from Prenticeboy
+1 to Nige. The Building regs frown upon bends in the wet part of the stack, and also require a (slow) 'rest' bend is used at the foot of the stack. If you want to move the stack outside, then you need to dig down and make a correct connection to the drain externally and block off the existing drain through into the building at both ends, once this is redundant.
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