Carbon monoxide alarms are required in just about every type of dwelling in Minnesota, and it's been this way for several years now, but there is still a lot of confusion about this requirement.
General Requirement - This text comes directly from Minnesota Statute 299F.50:
Every single family dwelling and every dwelling unit in a multifamily dwelling must have an approved and operational carbon monoxide alarm installed within ten feet of each room lawfully used for sleeping purposes.
One and one half story houses: you need a carbon monoxide alarm on the second floor. A carbon monoxide alarm installed on the first floor within ten feet of the stairway to the second floor does not count as being installed within ten feet of the second floor bedroom. The second floor bedroom begins at the top of the stairway, not the bottom.
What constitutes ten feet: Measure from the door of the sleeping room to the carbon monoxide alarm. If you have to pass through a wall, floor, ceiling, or doorway with a door that can be closed, it doesn't count. No carbon monoxide alarms in bathrooms.
How this law gets enforced: For the most part, it doesn't. Building inspection departments may notify homeowners that carbon monoxide alarms are required, but the intent of this law wasn't to make building inspection departments enforce carbon monoxide alarms (although many still do). The exception to this is Minneapolis and South Saint Paul; both of those cities require carbon monoxide alarms for their Truth-in-Sale of Housing programs.
Definition of "installed": this text comes directly from the Minnesota statute:
"Installed" means that an approved carbon monoxide alarm is hard-wired into the electrical wiring, directly plugged into an electrical outlet without a switch, or, if the alarm is battery powered, attached to the wall of the dwelling.
While the official definition doesn't mention mounting the detector on the ceiling, that's ok too. If a carbon monoxide alarm is sitting on someone's desk, it's not installed.
When to replace: Approximately 99.3% of the CO alarms that I come across are made by Kidde or First Alert. Kidde CO alarms last seven years, while First Alert CO alarms last five years. You won't find that information published on either of their web sites though; you actually need to call them to get that info.
Where to mount carbon monoxide alarms: follow the installation instructions from the manufacturer. In general, carbon monoxide alarms can be mounted high or low on the walls, as long as children can't mess with them.
The CO alarm pictured above was actually hanging from a cable jack on the wall of a kid's toy room; I'd consider that a poor location.
Reuben Saltzman, Structure Tech Home Inspections