The number of CO monitors required in any given home (new and resale), and the location of those monitors, is controlled by regulation adopted through the state building code. WAC 51.51.315 provides:"... an approved carbon monoxide alarm shall be installed outside of each separate sleeping area in the immediate vicinity of the bedrooms in dwelling units and on each level of the dwelling and in accordance with the manufacturer's recommendations."
The regulations leave some discretion as to where, near sleeping areas, the CO alarm is located. There is no requirement that it be placed high or low on a wall. But, the regulations are absolutely clear that in a two-story home, there must be at least two alarms and more than that if there are more than two levels to the home. If bedrooms are spread out across one level of the home, then it may be necessary to have more than one alarm on that level of the home. The exact location of the alarm(s) on a given level of the home is left, somewhat, to the discretion of the property owner. The requirement that at least one alarm be located on each level is mandatory.