@Margie Pierce From my perspective as a Pittsburgh-area home improvement contractor in good standing with the PA Attorney General's office, with one million bucks worth of insurance on every job I do:
The bathroom and kitchen you're planning are what you should be really worried about.
The city of Minneapolis issues plumbing permits and licenses plumbers -- it says so on their website. The city of Pittsburgh proper and the boroughs of greater Pittsburgh do not -- plumbing permits and licensing plumbers is handled by the Health Department of Allegheny County, where Pittsburgh is located. We do 3rd party paid electrical inspections, and everything else is handled by local code enforcement and the quality of that varies sharply across the boroughs. It's a mess, except for those plumbing permits. We all mind our P's and Q's when it comes to those.
As Napoleon once put it, "In this country, it is wise to kill an admiral from time to time to encourage the others." The health department has a ferocious reputation for going after small offenders and issuing draconian penalties when they find them. I'd bet a significant sum that the city of Minneapolis has much the same reputation when it comes to plumbing permits. Illegal sanitary water work destroys property, hurts people (typically children and the elderly), leads the news, and makes top local politicos look like they can't get their thumbs out.
So the problem with not pulling building permits (with special attention given to plumbing permits) if you're in this business for keepsies is that sooner or later, the odds catch up to you, and once they do, your name is mud and your career as a landlord is over. I maintain the absolute best possible professional relationships with the building inspectors who typically check my work, but I am ready to drop and shine an Allegheny County plumbing inspector's shoes if he wants me to. Over time, several have come to trust I will always do the right thing because I know the score. This makes my life easier in turn.
The simple construction reality is that if you sister your 2x8s, as indicated elsewhere in this thread, you can do whatever you want up there and the floor will hold. The plumbing work to cut into the stack and put in the fixtures you talked about isn't impossible hard even for a DIYer. But my recommendation is that before you do anything at all, call the plumbing department and indicate your intentions, call the office responsible for issuing building permits in your area, AND DO WHATEVER THEY TELL YOU TO DO. Or hire a contractor who knows the score. Pulling permits is more than a bit stressful but is an absolutely integral part of being in this business -- you've got to learn the process sometime. As an starting investor who owns a single duplex (as indicated on your profile), with the desire to get a few more, this is the best time to learn even though, perversely, it's the time when you can least afford the expense of it.
Politicos LOVE shafting and shaming us, Margie. If they could shaft one of us every day and give a speech about how "the good people of our fine city need not worry about predatory slumlords doing their tenants wrong on this administration's watch," they'd stay in office forever. Perp-walking one landlord in handcuffs on camera is worth kissing a hundred full-diaper babies for a politico. There's no downside for them, whatever party they belong to.
Good luck to you, whatever you choose to do.