Do you honestly think anyone would construct all the bell slab footing and vertical outside wall surface I described without connecting it to the footing that was originally poured down there?
Oh hell, why not? The slab might not sink or crack from driving several tons of vehicles back and forth over it every day. Why waste $60 of concrete to let the foundation footing support it, like it does the rest of the house?
The object of building a house (we DO try to keep this a secret) is to make the house last, NOT to try to save a few bucks by choosing not to support a big, wide garage opening with a footing already there.
"True eloquence consists in saying all that is necessary, and only that which is."
François Duc de La Rochefoucauld
yes, there is typically a foundation under the front wall of a garage (where the doors are located), see post #4.
the depth of your footing/foundation is determined by any required frost protection depth, or minimum depth as required by your building code.
The basic requirement from the 2009 International Residential Code (may or may not apply to your area) requires your footing/foundation to be at least 12" in depth, or to the depth of the frost protection requirement of your jurisdiction, whichever is greater.
"You get what you pay for, and sometimes free costs more!"