I'm sorry, but you need to do a lot more research, and not in newsgroups,
before you try any of these projects, or hire a contractor. Your local
codes enforcement office would be a good place to start. They'll not only
tell you what is required in your area, but they'll also tell you why. Many
areas of the country now have to consider potential seismic damage as well
as frost heave when planning structures, and that can govern how the
footings are constructed for various projects. Retaining walls are NOT
something for an amateur do it yourselfer to fool around with unless they
completely understand soil drainage and proper construction techniques.
Your life may depend on whether or not things are constructed properly. No
joke.