Is a sealed attic appropriate for Southern California?
If I were building the house in southern California then I wouldn't do this, especially if you were using asphalt shingles.

An unventilated attic in a warm climate like this can get extremely hot and will greatly decrease the life expectancy of asphalt shingles.  Over time they will start to curl prematurely from the heat.  If I were to do this I would insist the following courses of action:


Heavily insulate without sealing gables
Have at least a 1 foot overhang with ventilated soffits
If 1 foot overhang is not possible, then install an attic fan to assist in proper ventilation of the attic
If neither of these are possible then forgo asphalt shingles for something more appropriate for extreme heat like terracotta or slate shingles.