From the photo, it looks like you're not burying the waste pipes into the wall, but building out a soffit. Is that right? If so, did you decide to do it that way in order to save money?

There's just not enough room in the wall as I understand it...Do you think otherwise?

I can't tell from the picture. But it looks like your vertical waste pipe is fully outside the old plaster wall. Of course, if you wanted to partially bury it in the wall, you would have to break into the plaster, which would cost you money in demolition (but not that much). Cost of covering it up would be about the same because you are now paying to construct a soffit. You could have saved some space in the room if you partially buried it, but perhaps this wasn't a major consideration for you. Did an architect design this space?

and I would add, that there may have been other considerations that caused the designer to put it outside the old plater wall, such as the placement of joists or other obstructions.

I suspect it may be the latter (obstructions) rather former (cost). I'm going to look into it though...thanks.