If the wall is under a certain height- I believe either 30 or 36 inches no permits are needed either. Depending upon how steep your slope is you can pick up alot of space fairly inexpensively. My backyard is about 100 ft wide and my slope is average steepness. We built a 30″ sitting wall (no permits were needed and were able to push back about 10 ft into the slope adding approx 1000 sq ft of flat, usuable yard space. It made a big difference for a few thousand dollars.