As already suggested, a CO alarm (hardwired to also alert residents in the attic above) should also be installed (just in case someone decides to leave the engine running).
That's standard code. There has to be a co/smoke in the room above the garage for an occupancy permit.
I asked the BI about one in the garage but he said no because they can be tripped by a running car sometimes. The BI also said hooking the system to the garage doors was bad. You don't open the garage doors until your ready to go so if you warm up your car it's building CO until you open the garage door. If anything he said hook it to a CO detector or leave it running on low all the time.
He had no recommendations on a CO system but said we should ask our contractor. That's not happening since we parted ways on bad terms.
It sounds this is new territory for people so I'm a pioneer here. I'm sure I can muddle though it I was just hoping that wasn't the case. -- IF YOU FIND ANY MISTAKES IN MY WORK...Please consider that they are there for a purpose. I try to please everyone and there is always someone looking for mistakes!