Carbon monoxide detectors will only be required in existing homes that have fuel fired furnace or water heater OR that have an attached garage.
Link to info
Carbon Monoxide Alarms
Scroll down a bit when you open it.
From the commentary.
CHANGE SIGNIFICANCE: Carbon monoxide alarms are now required
in new dwelling units constructed under the 2009 IRC. Because the
source of unsafe levels of carbon monoxide in the home is typically
from faulty operation of a fuel-fired furnace or water heater, or from the
exhaust of an automobile, this new requirement applies only to homes
containing fuel-fired appliances or having an attached garage. Carbonmonoxide
accumulates in the body over time relative to its concentration in the air. Accordingly,
carbon monoxide detectors sound an alarm based on the concentration of carbon
monoxide and the amount of time that certain levels are detected, simulating an accumulation
of the toxic gas in the body. High levels of carbon monoxide will trigger
an alarm within a short time, while lower levels must be present
over a longer time period for the alarm to sound. This design prevents
false-positive alarms. This change in the code recognizes the improved
reliability of carbon monoxide alarms and the referenced standard, UL
2034, Single and Multiple Station Carbon Monoxide Alarms, and intends
to reduce accidental deaths from carbon monoxide poisoning.
Under the new provisions, carbon monoxide alarms are also required
in existing dwelling units. Similar to the smoke alarm provisions,
this requirement for installation of carbon monoxide alarms is
triggered by construction work on the existing dwelling where such
work requires a permit. Unlike the smoke alarm requirements, there is
no exception for exterior work or the addition of decks or porches.
Roofi ng, siding, window replacement, and other exterior work requiring
a permit will require the installation of carbon monoxide alarms.
Because carbon monoxide poisoning deaths often occur when the
occupant is sleeping, the IRC requires carbon monoxide alarms to be
located in the areas outside of and adjacent to bedrooms.