not trying to be vague but this is basically how it goes...
If its a garage that will be cheaper than regular because there is no finish work.
If its a ckitchen it depends on the cost of appliances and what type of counters and cabinets you install..
If its a bedroom than the cost usually runs from approx $150 per sq foot to $250 per sq ft
It also depends on wheter you have to re pitch the roof for the addition and present condition of the home.
Remodel can be more expensive than new because you don't know alot of things till you open walls and check plumbing and electrical.
If the home is on a hillside then you have foundation and and slide and seismic considerations....
NOT TRYING TO SCARE YOU!!! THIS IS STILL THE BEST INVESTMENT YOU CAN MAKE!!
RULE OF THUMB FIGURE IN SARATOGA WITH THE FACTORS I LISTED FRO APPROX $200 TO 350 PER SQ FT DEPENDING ON BUILDER AND QUALITY AND FINISHES....