Hey, I thought it doesn't rain in Southern California
so there's nothing to worry about.
From your drawing it looks like there is some sort of drainage easement between the two properties specifically for this situation. In NJ, we normally pitch our patios from 1/8" per foot to 1/4" per foot for runoff. This keeps the water from sitting on the patio and the olives in the martini don't sit to one side.
I agree that it is worth meeting with a contractor or landscape architect, or engineer to discuss a solution that may well be more cost effective.
Good luck with it and please keep us posted.