Retaining Wall for Garage Home Addition

I'm planning to build a home addition for a client which will be a garage to house a camper and boat. This project will involve constructing a retaining wall that is 20ft wide x 6.5ft tall on back end then level out on sides from 6.5ft to ground level. The ground runs downhill from the front of the house, (shows one block 12in in the front) and at the back end is 6.5ft from top of block. The dimensions of this garage home addition are expected to be approximately 18ft x 40ft. At this point, no specific features such as electrical outlets, overhead storage, or plumbing connections are needed. I am more concerned on how the wall should be built to handle the weight and have the proper drainage!

Ideal candidate should have:
- Extensive experience in constructing retaining walls
- Knowledge of garage home additions
- Understanding of soil erosion prevention best practices
- A good eye for landscaping aesthetics.