Interesting question. I'm not sure about your local code, but the average garage door is only 7 feet tall. If you build the garage forward 20 feet where the surface is rising rapidly, you won't be able to park a car in the garage, and its going to look like a mistake anyway. Also, any rainfall would sheet straight into the garage. Common sense dictates you do need a flat landing in front of the door whether the code says so or not. Like I said, be ready to duck.
Do you have room to extend the garage out back, or to the side, and how tall would the retaining walls need to be?