I am new to DIY construction and I have an existing 60x40 pole barn. I don't need that much barn, but want to build some living area in the back 12 feet (making the living area 40x12). The side walls are 18 feet tall I believe, so I would like to make this living area 2 stories tall to maximize the space. The areas I would like to build are an office, a full bath, a bar, a bedroom, and a studio. The barn has 100 amp electric service and lines for propane and water. I plan on contracting out the plumbing because I don't trust myself with that part. The construction and electric I am doing myself. The question I have is how to start. At this point I have graphed out my rooms for both levels keeping in mind the location of the water source and where it will be used in the new rooms. Do I bring in the plumber before I start laying the framing? Do I frame everything out first? Is there a good "construction project walkthrough" online that would be of help? Thanks in advance!