The building code lists several kinds of structures and work exempt from permits and inspection. One of the items on the list may sometimes be exempted and sometimes not. Retaining walls may or may not need a permit. Whether a retaining wall is exempt depends of several things. Building Codes Division has issued an interpretation to try to clear up any confusion. Here is a summary of that interpretation.
Some retaining walls must always have a permit and undergo plan review and inspection and are never exempt. Retaining walls which would impact buildings, parking and exiting required by the code are not exempt. Here are some examples of non-exempt retaining walls. A retaining wall which supports material that supports a building must inspected to be sure it provides adequate support for that building. A wall close to a building which holds back earth that would fall against the building should the wall collapse requires a permit. A wall supporting a required parking area for the building is not exempt. Walls which support or could fall on a required path of travel are not exempt, such as the walkway from the entrance of a house to the street.
Retaining walls which do not require permits are walls which do not impact a building regulated by the building code. Examples would include walls that only used for landscaping; walls on a property line that would not impact the building on that property if they were to collapse; retaining walls that protect a private road; and walls that protect a non-required walkway in the yard. None of the exempt walls would cause damage to the building or create a hazard for people leaving the building should they collapse.