Well Darren, You could do a step down to the garage but then it will always feel like a converted garage because of the step. But leaving an 8" floor may not be enough for the plumbing and duct work.
Bathrooms, lighting, etc. are going to need extra circuits and wiring. It is not a matter of adding a few outlets and moving some down.
Your washer has at best a 2" drain and you need 3" for a toilet so you better look for the closest 3" line to hook into. You may also need to install a cleanout. Your washer is also probably plumbed with 1/2" water lines and you will want to take off starting from 3/4" lines and run 3/4" all the way to your new tub/shower and branch off with 1/2" for the lav and toilet. Unless you have a big family, the 55 gal wh should still handle an average size whirlpool tub which if you put one in was also one of the extra electrical circuits to run I mentioned in my other post.
If your septic system handles your crap now, then I wouldn't worry about it. Anything the city would do would be outside the house anyway. But if your plumbing, etc in the house isn't up to code, they could make you correct it before they hook you up.
You really need a plan drawn up. Too many people start building and then find out the plumbing and hvac ducts can't be run like they need to be and then it costs more to fix. If you don't do everything to code, it will cost you "bigtime" to possibly have to tear out and replace before you can sell it. Good luck and watch out for wild artillery rounds from Ft. Bragg.