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!

Got any pictures?
First thing I would do is see if you can even get a permit to build it.
May sound personal but do you have the cash to build this? Reason I ask is I highly doubt any banks going to lend money to someone who has never done this before.
Does this building already have a slab?
If so it may be more of a liabilty then an asset. There's needs to be footings under all the supporting walls, all drains and water supply will need to be run under a slab.
There also needed to be a vaper barrier and insulation (foam panels) installed under the slab before it's pored.
If the slabs not flat and in good condition it's a nightmare trying to come up with a way to work with it when it comes to flooring.
There's at least 100 other things to concider.
Since this is your first time I would strongly suggest you at least find a real builder to work with you one this plan, better yet have them rough frame and dry it in and you work on the 100's of other things that will need to be done.

I don't have any pictures yet, will work on that. Permits aren't an issue in my area and situation. I have enough cash to build most if not all of it, and can always finish it out at a later date but would like to at least get it framed and set up for the plumbing if that is the order you suggest.
the building already has a slab. Water is already run through it. Is there anything I can do post-slab to satisfy these requirements?

My first concern would have been the permits, especially since you're combining an outdoor structure with living space, but sounds like that's not an issue.

You might also want to check with your homeowners' insurance carrier to make sure that they'll ensure this combination of outdoor and indoor construction.

your gonna need a 200 amp service
the first leval is poured already ?
if u want plumbing on 1st leval, you gotta bust the floor.[cut concrete] $$$$$$
or install a floor above the concrete. either way'
plumbing 1st, then framing,plumbing again after framing then roofer, elec,hvac,cable,windows doors, insulation. sheetrock,tape float, paint, cabinets,cabinet tops,plumbing,elec,hvac,flooring
mirriors,case trim, touch up paint
landscaping, driveway,sidewalk,etc...
he(), i know i forgot somebody !!!

Siding