Fire Separation at Exterior Deck to Adjacent Building

[jrobinsonarchitects]

Hi! I've got a client in San Diego with a three story live/work loft building that would like to utilize a third floor mechanical roof space as a deck. This is not the building roof and is surrounded by walls on all four sides but open to the sky above. This space is at the back of the property against the adjacent building and there is zero setback at the property line. What will the requirement be for fire separation from the adjacent building for an outdoor space?