Retaining walls retaining more than 4 feet of unbalanced fill (bottom of footing to top of retained soil) require PE-stamped engineering per IRC R404. On the Wasatch Front, SDC D seismic requirements add a critical dimension: retaining walls must also resist horizontal earthquake forces in addition to static soil pressure — seismic design is more demanding than static design and is required for essentially all retaining walls over 4 feet in Sandy, SLC, Draper, and other SDC D communities. Retaining wall footings must be placed below frost depth (30–36″ minimum in SLC valley). Drainage behind the wall — drainage aggregate and perforated drain tile — is an engineering requirement, not a choice: hydrostatic pressure from inadequate drainage is a primary cause of retaining wall failure. Walls over 4 feet are structurally significant — engineering is not optional.