I’m adding a single-story room onto my house and need to know how deep to make my footing. I understand that it should be below the frost line (12″ in my area) but does that mean that the entire footing should be below 12″ or just that the bottom of the footing needs to be that deep?
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The bottom, since that is what you want to prevent from heaving. The very best is to check the prevailing standard in your area. Furhter more, a building permit will probably be required because, in all likelihood, you will be altering a load-bearing exterior wall in the existing house (putting in a door). "An ounce of prevention...."
If you use block for the foundation, figure the depth backwards from where you want your flooring to finish out in relation to the existing flooring and your local code.