In Southern California, during new construction with a finished garage, the cost of adding insulation at this point should be low.
But if the space will not have any active cooling or heating elements (outside of a tankless water heater and, perhaps, a cat), is there any value in insulating it?
Everyone loves going from a cool house in to a hot garage or vice a versa. Would insulating it make any difference? Would it not get as hot in the summer or as cold in the winter?
Seems to me that it would retain whatever temperature it have longer (whether hot or cold) with the insulation than without, and one could "reset" the temperature of the garage by opening the door for a few minutes to the outside (for example, open it in the cool morning in the summer hoping to better maintain that during the day). Just not sure if it would be enough to notice or bother with.
Historically the garage has mostly been closed vs opened several times a day for cars. Whether that will be the case at the new house is up for debate.