What is the new CO and Fire alarm law for Austin, Texas?
@Andrew Postell probably knows the answer to that Texas-specific question.
Each bedroom and if multiple bedrooms per hallway, in hallway. No requirement on Carbon Monoxide detectors.
Unless Austin has city-specific requirements, the state requirement is one per bedroom, plus one in any common hallway shared by bedrooms, plus any common areas like a living room. There's no specific requirement for carbon monoxide alarms.
While we're on the topic of safety devices, the property code also requires a peephole on every exterior door and a keyless deadbolt or latch on every exterior door.