[fintstone]

Smoke detectors were wired everywhere in my newer homes (pretty much every room and hallway) and replacement combination CO/fire were designed to swap right in with same base/wiring. It takes a bit more thought for the older homes and the wiring is not there. In most places, the requirements for detectors in existing homes in general (and more so for CO) are quite low (although you can add ones that work on battery alone (wired ones use a battery backup). I have had to meet code for rental homes (when I changed the use of the home) ...but usually try to far exceed the minimum. I like the redundancy theory...even in totally electric homes (which I greatly prefer for rentals).

Most folks should check the expiration dates on theirs as they lose effectiveness. It is usually on the back or in the battery compartment.