How to build a foundation on two levels?

[skikir]

I'm planing on building an addition but to start out I want to do an "as is drawing" as a place to start. My house is on a slope and has two levels with the garage on the bottom and the house above that. The house is larger than the garage and spreads west above grade of the slope. So I have essentially a day light basement for the garage. I can't get started on how to add the second floor and add a foundation to the second floor above and to the side of the garage. The garage has a concrete retaining wall on the west side that I can't seem to get set on the foundation either. Please see attachment.