I could repeat the stay-away advice of Henry in MI and BEG and others in my own words but I won't at the expense of sounding repetative. But I will add that even the most confident Do-It-Yourselfer (vs. a professionaly) is apt to have a lot of questions when it comes to a job you're describing. If this is the case, it's a no-brainer IMHO. Don't do it. If you bought a book for a few bucks at a bookstore that writes about, say, converting a garage to a bedroom or to a den, you'll see what's involved. THAT should be enough to discourage you.
Now, for the good news. There isn't anything that prevents you from spending your $$$ wisely by having a pro do the job BUT saving your $$$ by doing the finish work yourself. You can paint and stain I'm sure. Heck, you can even make a party out of it! (Just make sure you invite a friend(s) who know a little about the 'job' you're doing so they can help/advise you on details.) You can buy all the necessary electrical fixtures for you new room too. You needn't have the electrician do it. The same goes for the flooring. Have a pro do the 'rough' work for the type of flooring you want to put in and you can put in the parquet. Again, $$$ is saved! (I wouldn't carpet yourself. Do the parquet and put down an area rug.)
Another reason besides those already mentioned as to why you want a pro to do the rough work is resale of the home. When you go to sell the home, if any type of inspector is involved, more than likely he/she will see all the mistakes. (It's their job.) No heating ducts, no A/C, no insulation, random outlet randomly/wrongly placed, condensation on the walls, etc., etc.. And, when the potential sellers ask for the permit for the 'renovation', what are you going to do? Perhaps the inspector will notify the municipality in which you live of the fire code violations, or habitat violations. I always advise folks that whatever they buy a home or whatever they do renovation work, think of the end-result in terms of when you sell your home. You may say now that you're not selling ever, or you're not selling for 20 or 30 years, but you WILL have to sell if you can't pay the mortage and you're in forclosure and the bank wins! (Well, I guess that theory is out the door if you own your home outright and you're independently wealthy ...) Anyway, there are a number of circumstances that could 'force' you to sell, and at that time, this DYI project could haunt you. And one last consideration ... IS the 'type' of renovation you're doing going to help the resale of your home, or hinder it? Who would be interested in your home once it's finished? Folks having trouble doing stairs (since you have a 1st floor bedroom now)? A single lady never planning to marry or have children? And so on. I've seen homes where the owners were, say, elderly and having trouble 'getting around' and they converted a garage to a bedroom because they couldn't do stairs any more. The VAST majority of the people looking to buy a home are young, married (or soon-to-be), and childless. They don't want a 1st floor bedroom, NOR do they want to do a major remodeling project to 'convert' a bedroom back to a garage. I DO know that a lot of single adults these days are buying new townhomes, and a lot of elderly retired folks are buying/renting single floor, maintenance-free units. The latter are NOT buying 2-story homes with 1st floor bedrooms! Not even for their grandchildren to visit! (It would mean they'd have to go up and down to clean and such, and this is when they AREN'T visiting.) I think you get the picture ...
This is my $.02. My best to ya and hope this helps.