Around here the cost is about $110 to120 sq. ft. for single story house, to go up its about $95 to $105 a little cheaper to go up. You have been given some really good advice so far .
1. check the structural integrity of the house will it hold the added weight?
2. If you are not ever planning on selling your house do you want to climb stairs at age 65 or 70?
3. With the advances in building tech adding on to a brick home (same level) is a lot simpler and can be done quite well now

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The mind never forgets what the hands have learned