The city shall ministerially approve an application for a building permit to create within a zone for single-family use one ADU per single-family lot if the ADU is contained within the building envelope of (1) an existing legally established single-family residence, or (2) an existing legally established accessory structure, including, but not limited to, a studio, pool house, or other similar structure, that has independent exterior access from the existing residence, and the Community Development Director determines that the side and rear setbacks are sufficient for fire safety.