Having lived through a major addition to our home, much of what has been said by others has lots of value. First check with your local zoning and planning boards regarding the addition. Also the building permit officials.
We tore off one room from the back of the house and added to the entire width. With the increased square footage, we additionally had to upgrade the electrical system for the entire house. Also the smoke/co detectors to (then) current code. Because the original portion of our house is 200+ yrs old, also had to add lots of exterior decking to meet (then current) code for egress. And since we live in NJ - the 'if you've got it, we'll tax it' state, that increased our property taxes significantly. (Exterior decks are considered 'living space' here so you get taxed at the same rate as the interior of your home). We also had to either add another heating system or upgrade our existing. We decided to go with a new system to accommodate the entire structure vs 2 separate systems.
All of that said, it was still the correct decision for us as the house was just too small otherwise to live comfortably and buying another was even more costly on a monthly basis. Just do your research carefully and exhaustively before making final decisions.