Re-plumbing on edge of slab

[John Eskew]

I have a bathroom that was framed up nearly 2 ft off the slab foundation with two steps upon entering. A hole is cut in the slab beneath toilet for tub sink and toilet to connect to drain line, but shower and sink drain lines run on top of slab under framing. I would like to lower the bathroom to slab level but am having trouble finding a solution for tying into venting on the edge of the slab through a 2x 4 exterior wall.
Same as below configuration, though on edge of slab foundation:

Any layout suggestions would be much appreciated

 

[Tuttles Revenge]

Horizontal wet venting will be the way to go. On the floor plan layout above, where is the point of connection where the drain needs to connect?

Below are 2 options that work for horizontal wet venting in UPC with washington state amendments which don't require the toilet to be the most downstream fixture. The sink drain in the middle of the right most drawing is to keep the trap arm of the tub to its required length.

 

[Sponsor]

 

[John Eskew]

The tub, sink and toilet drains come together in a cross t under the toilet then enter main drain which runs perpendicular, from under toilet to septic tank. The vent enters the wall on top of the slab behind the toilet- no dry venting from tub or sink. I’m concerned about cutting the slab right on the edge to get the vent pipe through bottom plate. Would I have to enter the wall over the slab level? Maybe hide this behind tub?

 

[Reach4]

The tub, sink and toilet drains come together in a cross t under the toilet

You were not expecting that "cross t" to be in use after your changes, were you?

Also, if it helps, you may be able to route pipes outside of the slab.

 

[John Eskew]

I’m guessing I should change that
Would something like the following work

except in my case the main drain runs perpendicular to back of toilet
And I could route bottom of sink drain/vent around to the outside of the slab to avoid cutting the edge of the slab