Flexible gas lines behid walls, up to code?

I will be having a plumber doing some work in the garage for the heater. He tells me that he will be installing some flexible gas lines which are being currently used in new constructions and are approved for use behind the walls (not under the house). He tells me that this is much easier to install therefore it would cost less. I just want to verify that this type of flexible gas line is up to code and can be used behind the walls. I live in Southern California, so he said that this is a big plus specially when it comes to earthquakes since the lines won't break. Any info would be apreciated. By the way this is a licensed plumber if it makes any difference.