As of July 1, 2011, owners of single family homes that have a fossil-fuel burning appliance, fireplace, or attached garage are required to install CO alarms per California State Law SB 183.

Applies to multi-family dwellings on January 1, 2013.
Does not apply to all electric dwellings that do not have an attached garage.

Manufacturers recommend placing the alarms outside sleeping bedrooms, and on every level, including the basement.

Senate Bill No. 183, chapter 19 states the following:

17926. (a) An owner of a dwelling unit intended for human occupancy
shall install a carbon monoxide device, approved and listed by the State
Fire Marshal pursuant to Section 13263, in each existing dwelling unit
having a fossil fuel burning heater or appliance, fireplace, or an attached
garage, within the earliest applicable time period as follows:
(1) For all existing single-family dwelling units intended for human
occupancy on or before July 1, 2011.
(2) For all other existing dwelling units intended for human occupancy
on or before January 1, 2013.
(b) With respect to the number and placement of carbon monoxide
devices, an owner shall install the devices in a manner consistent with
building standards applicable to new construction for the relevant type of
occupancy or with the manufacturer’s instructions, if it is technically feasible
to do so.

Check out this special offer from First Alert for a discount on an alarm through 8/1/11
http://www.firstalertstore.com/store/california-carbon-monoxide-alarm-offer.htm

Skys Property Inspection Consultants are in no way affiliated with First Alert and do not warrant their products.