The Washington State Building Code Council, RCW 19.27.530 requires that CO alarms be installed in non-owner occupied, existing dwelling units by January 1, 2013. The language of the regulation is unclear but many experts interpret it to mean that all rentals must have a CO detector whether or not there are fuel burning appliances or an attached garage. The law is also unclear as to the number of detectors required. The safest bet is to install them per new construction requirements. It is better to have too many than not enough! We also recommend detectors that are attached rather than plug in monitors as the plug in variety tend to disappear.