Yes, there is a 4' retaining wall on two of the three sides I'm adding on.Royce: you probably should start a thread in Garage Gallery to show off all the cool stuff you make and with all the cool things you build even do these smaller threads here. this section doesn't get as many member's or lurkers eyes for some reason and millions seem to see Garage Gallery section.
your choice for certain and just making another suggestion.
just wondering if you are making that 3 foot cement footing in styrofoam because the back side of your shop has some land sloping up behind it? great space you are adding on both sides too and happy to hear your weather isn't too bad yet.
good luck and hope you get it weathered in before the snow season hits your area which i'm guessing is any day now.
cheers