My name is Ken and I currently have an expansion on my house about 500 sq ft and have been considering closing it off and making it into a separate apartment. My only concerns with this is plumbing, I currently work in HVACR field and am familiar with construction, but don't know if this is something beyond me. My plans were to install this new bathroom right in front of an existing bathroom, that way it'll give me a straight shot to tie into drains and water lines. The only thing is that I would have to penetrate a masonry exterior wall to bring any type of lines in being that my roof goes from pitch to flat at this point. Is this even possible and if so what would I have to do and how hard is it?

Hi Ken,
Is your house on a slab or do you have a basement? Regards, Tom

It is on a slab.

Most bathroom groups are roughed in like this.
Toilet connects to sewer main. The toilet wet vents through the lavatory vent. Lavatory connects to toilet drain and runs a vent out the roof or revents back into a dry vent in the attic off the top the stubout tee. The shower connects to the lavatory drain and is wet vented by it. This is a normal rough in and is acceptable both by local and state codes but also The Standard Plumbing Code Book.
You will have to jackhammer up the floor to install the drainage. I don't understand what the roof pitch has to do with the drainage. If the addition is outside the original foundation then can you conne4ct the new bathroom to the sewer line? Regards, Tom