Effective April 1st, 2012 (not an April Fools joke!), Washington State Law (RCW 19.27.530) requires the seller of any owner-occupied single-family residence to equip the residence with carbon monoxide alarms (commonly called “CO alarms”) in accordance with the state building code before a buyer or any other person may legally occupy the residence following the sale. This requirement applies to all single family residences, including single family homes, condominiums, and manufactured/mobile homes.
The building code (WAC 51-51-0315) requires that an alarm be installed: (1) outside of each separate sleeping area in the immediate vicinity of each bedroom; (2) on each level of the dwelling; and (3) in accordance with the manufacturer's recommendations. The building code also requires that single station carbon monoxide alarms comply with UL 2034. There are no exceptions for properties that do not have fuel-fired appliances or an attached garage. The alarms may be battery operated and can be purchased for as little as $18 from a variety of sources. Here are a few examples that I found at Home Depot and Lowe's:
I found several types, including combo units that do double-duty as both fire and carbon monoxide alarms. Some even sound out voice alarms, if that's your preference. Consumer Reports has a good Buying Guide for CO Detectors, but they want you to subscribe to their website before they give you their recommendations. First Alert and Kidde seem to be the most popular brands and they each have a variety of models to choose from. Decide what you want before you go shopping! Or you can pick them up online at Amazon or other web retailers.