We added on a kitchen and did it all ourselves from the ground up. It's a good thing, though, that it's off the back of our house, sorta in the country and hidden by shrubs b/c we didn't get any permits
SHHHHH! DH knows all the trades, so even if we had an inspection today, it would more than pass (think: Mr. Monk the Electrician
) If you can manage all the permit and inspection stuff and have the patience to do it, then I say go for it. I never thought I'd help raise a ridgepole, but I did it! You will have to hire subs for some things (electrical, for instance), but a lot you can do yourself if you're the slightest bit handy (and after seeing your new living room, I'd say you're in good hands!)
Just remember...measure twice, cut once! And...the foundation is the most important part of the home.
Good luck!!!