I should have also mentioned I owned a house with a 15 foot high slope in the back yard for 20 years. The back yard was probably 30-35 feet deep and I added a couple of feet of depth by building a 30 in high retaining wall myself and facing it with stacked stone. That was my project for the summer. Info on here is correct – no permit required up to 30 inches.

I had to ensure my back yard design allowed for adequate slope away from the house and away from the hill and eventually got it all sorted out. I first tried a perforated drainage pipe and I can tell you that is not a good choice. Switched it out for large solid pipe and problem solved. Never had another issue nor gave it another thought.