I'm thinking about adding a dormer bathroom to my 100 year old house. It'd be situated above the existing bathroom, and I think I can bring the main wet wall up. I'm not a licensed contractor. I've done some building and tons of DIY home repair and improvement... but this would be a large project. I'm thinking a flat roof (it'd mirror the dormer bedroom on the front of the house) about 12 feet wide, sink, toilet (tap into the soil stack below), shower...

A friend also suggested the possibility of having a pro builder provide plans and even frame it out, window, rough plumbing, leaving the finish work to me. What do you guys think? I have all the tools I'd need, and plenty of time on my hands... what would you do?

Thanks!