(sorry for the long reply - some may be useful!)
I live in Davidson County and had to build on an addition onto our house several years ago. Here's how it started:
My next door neighbor used to be a contractor. He and I were talking about how small our houses are (3 BR 1.5 bath) considering both of our families at that point had two teenage girls who constantly needed the main bathroom. I asked him how hard would it be to put a stall shower into the half bath and since it is like 6x6, it's near impossible. So he suggests building an addition off the front of the house to serve as a master bedroom suite. We think towards 15x15. During the conversation, he suggests "while we are building it, we might as well add a little more to give me an office". So we think 15x20. Then my wife butts in and says, "if he gets an office, I get a walk in closet". 15x30. Oh... and she wants a jacizzi tub for the bathroom (might as well enjoy it!) So we end up with an 18x36 master bedroom suite in the plan of which 9x9 walk in closet and 9x9 office. My contractor neighbor all the while easing my nerves saying he will help with everything.
We take the plans (simple sketches on graph paper which lays out where the existing house is, the road setback and how big the proposed addition is) to the county for review. They send out someone from the Health Department to review the septic and puts the brakes on the whole thing. Seems as how our septic is only rated for 3 bedrooms and we wanted to add a 4th. He says he has good news and bad news...
Bad news is we can't add another bedroom. The land we are on will not perk to today's standards to add more septic capacity. With that kind of bad news, I wonder what kind of good news he can have...... Well, we can build a "bonus room". I ask him if the bonus room can have a bathroom attached? Sure. Can it have a whirlpool tub in it? Sure - big as you want. Just can't be a bedroom.... (so no closet!!!!)
A quick change of the proposed plans has a Bonus Room with a bathroom with stall shower and whirlpool bath (6' so that if I ever want to enjoy it I can, too!) and TWO 9x9 offices (his and hers).
I secure all the permits for plumbing, structure and electrical and my HVAC contractor secures the HVAC one (we added central air to the whole house at the same time). We get the foundation poured as I'm watching a moving truck pull up to my neighbor's house. He didn't tell me he was moving. There goes my contractor neighbor and any hope I had of getting this thing done. Or so I thought.
I am not a framer, carpenter, electrician, plumber, or anything in the building trades. Before I started on this, I didn't own a nail gun or a square. I had the internet and a basic idea of what I wanted to get done.
The guy who poured the foundation suggested a mason for the block and brickwork for the crawlspace. Turns out the guy wasn't as good as he thought because the block ended up about a half inch narrower at the road side as the house side. This didn't end up a big deal since I adjusted for it in the floor joists and made it square overhanging the brick by 1/4" each side. Only I would notice really.
One thing I didn't notice at the time was that the existing house does not have a sill plate (a weather barrier between the block and the joists.) I don't know how it got past the inspectors in the 70's when the house was built, but all the houses on the street are the same. Current code requires a sill plate or a bituminous membrane between the block and the joists. In order to get the house and the addition to line up, I had to rely on the inspector and the guy from the building supply company for suggestions. They found a thin vinyl/plastic barrier that worked for this membrane and we were off again. I was very lucky to work with a guy at GW Smith lumber who knew how clueless I was, but really wanted to help.
The floor joists are one thing that I wish I could have done differently. Because I was spanning 18 feet, I had to go with some pretty tall joists. I think they are 12". I had to space them every foot because of the span. When the foundation guy was pouring it, he asked if I wanted some pillars to help break up the span. I declined since I was within spec based on teh quintity and thickness of the joists. In retrospect, the pillars would have given me less creaky floors and less bounce, and as it turns out wouldn't have cost anything since he already had the materials on hand. Lesson learned.
I laid the subfloor and began building walls. Not knowing anything about framing, I learned about stud spacing the hard way. I thought I'd be crafty and build wall segments in 8 foot increments so I could move them around myself. (did I mention that my wife is pregnant during this point and most times I'm working alone?) The 8 foot walls were nice and convenient, but the studs rarely lined up with the wall sheathing, so I usually had to add 2x4's to fix gaps. Another lesson learned the hard way - but nothing impossible.
Once the walls were up, it was time to get the trusses built and installed. In my case, we only took out a loan for the amount of the foundation, roof and HVAC since they were the big ticket items that I knew I couldn't do myself and the roof and HVAC were for the whole house not just the addition. A LOT of time passed between putting the subfloor down and putting trusses up, so that meant a lot of time spent under a tarp. By now, my daughter is walking around while my wife and I are trying to figure out how to hoist these trusses up just her and I. The truss at the end of the house was the easiest since the bottom of it is the top of the wall. Line it up and nail it home. Support it from teh ground and she stays put. Now, the other ones aren't so nice. Since I wanted a vaulted ceiling, there are just the two outer points to connect to the walls and nothing to keep them from falling over while you nail them in place. Here's where I get crafty again. I decide to attach a 2x4 onto the truss at the center of the V so that when I set it on the wall, we can use the 2x4 as a lever to rotate them up to their upside down V shape on the wall, giving my wife something to hold on the ground while I run around with a ladder and nail gun (I bought one AFTER building the walls - another lesson learned). All was well until the zip ties that I used to hold the 2x4 to the truss snapped causing the truss to rotate back around nearly falling on my daughter. Enough of this lone wolf crap - I needed help.
I put the word out at my church and had a work Saturday setting trusses. Two guys came to help that were perfect for the job. They were both line workers used to heights and building stuff. We had all the trusses set by lunch and had half of the roof sheathing up by the time they went home. The other half of the sheathing was put on a little at a time between work and rain storms. By now, the tarp had large holes in it that allowed a lot of water to hit my sub floor. Not good, but not much I could do at that point. It dried, and I had to do the best I could to get it level again. Another neighbor of mine who I had never talked to came over to see how I was doing. After a beer or two, he explained to me he was a contractor and that the method I was using to tie the old roof to the new roof wasn't going to pass inspection. He told me what to buy and it was there the next morning as well as his crew of guys to finish the roof tie-in. He never charged me and he has been a great friend to have ever since.
He also pointed out a couple small things that I needed to fix prior to drying it all in like one of my window openings didn't have enough studs around them for their size. My door openings and the smaller window openings were OK, but the large opening needed more beef.
I had a couple run-ins with the inspector mainly due to plumbing. There's not a lot of online help when it comes to rough plumbing. All they tell you is "Hot's on the left, cold's on the right and poop don't go uphill." I did learn the hard way not to "co-mingle" ABS and PVC, even though there is a specific green adhesive they sell at Lowes to do just that. The inspector failed it even after we looked up the ASTM requirements from the side of the adhesive bottle. I had to cut out the section and replace with ABS (I would have done it the first time, but Lowes was out of that material that day - thus why the salesman suggested the adhesive).
I had an electrician (WillC from on here) do the rough electrical and upgrade my breaker box at the same time. He was AWESOME. Unlike the original house wiring, I know everything is done right and he explained what everything was, and why it was there. Electrical was one of the few things to pass first time every time.
As I was hanging the sheetrock, (roughly 5 years after breaking ground) I watched as my contractor neighbor moved BACK in next door. He was very impressed at the work I had done. It's nowhere near professional quality, but honestly....the original house wasn't either. But, I can tell you why every nail and every screw is where it is - because I put it there.
Seems every single project now days runs into something stupid. There again, Engineers can be golden. Good luck!
Hell, the sky's the limit now. 
Forget everything I said and call and talk to planning about building another house on to yours. Seriously, a duplex. Dude, sewer is night and day compared to septic. And good chances are you can rezone.