Is building your own house/addition documented step-by-step anywhere?
There isn't any easy advice for this.  I am sure there are books and resources that cover this but it is highly regional and highly dependent on the type of addition that you want to do.  

For instance the most common things I have done is chop the roof on and add another level - works for ranch houses with no one living in them for 2 months - and the most common is convert garage to the house and add on another garage. 

We can add a garage for 25K most of the time because of how cookie cutter the experience is.  There are several companies and my area that will drop off everything that you need to frame your garage with architect's drawings.

As for quotes I wouldn't even pay attention to people that quote per sq/ft.  What the hell does that mean?  $165 sq/ft - great I will take gold trim.  As a home owner before you go to a GC I would:


Have rough drawings of addition
How the addition's roofline will integrate with home
I do not suggest hitting an architect yet - unless the home in question is rather expensive or very architectual (I know that isn't a word but you know what I mean).
Get the list of things you need done.  Foundation, framing, roofline/trusses, intgration, housewrap, insulation, siding, whatever.   Have a complete list.
Take this to your local building inspector.  Make sure this jives.  You could even leave these as planning papers as you pull your permit.  The local inspector if helpful might give you pointers on foundation requirements, if the city will allow certain things, and so on.  This discovery phase will allow you to skip a lot of bullshit with the GC you hire.  Basically he can't bullshit you and you cannot ask him to do dumb stuff if this is done right.
When the inspector OK's the idea with caveat of proper plans/drawings then you go to your GC.  If your GCs don't have a "system" in place to handle your type of addition, then get another one or at the very least go to the architect and get drawings done before bids.  If your GCs have a system in place they might have prefabbed designs and maybe if you alter yours just a bit it can be the difference between 5-10k.
With the list of things you want done get a contract with the GC stating what and when things will be done with payment plan.  Also discuss if trades will be paid directly or by GC.  Same thing for materials providers.
Let me tell you something GC make the most money off of the finishing touches especially if they have a good crew.  I have 2 drywall guys that can drywall a 3 bedroom apartment (ceilings too) in one day and a half day to sand.  I could pay them $600 for that and charge you $1500 or more.  For big things like concrete work, foundation stuff, materials, roofing and so on there isn't as much room for a GC to make money.  Hence he will be working with reputable companies to do these things that you could also contract out (the cheap drywall guys that do a great job - you don't know them or they aren't allowed to work without GC involved).
So my advice is have a detailed list, drawings, and everything.  See what the GCs in your area want for the job.  Call some concrete/foundation companies, plumbers, electricians, and roofers and see what you think your costs will be.  Also be flexible on letting GC do finishing touches - just don't value those when negotiating.  If you say I don't want you to finish the job it might turn a lot of GC off.  But after they write a bid and you say that is too much, what can we leave out since I will be out of money?  Well they will react different to that.  
With a GC you are paying someone for organization, person that has connections to the right people/trades, person to catch issues before they are problems, and a person to manage others that you may never trust.  If something goes wrong you blame him.  Now if you do it yourself know that you will be working from home a lot, you will be organizing a lot, and your project will probably come together slower - since you have to have a buffer for things to get done.  Also note that the smaller trades may not perform as well for you as they would GC as you are a one time deal and the GC will throw them more work.  So even if you have the same people, more managing to do.