2012 International Residential Code:
Detached one- and two-family homes and townhomes (dwellings) of not more than three stories within which fuel-fired appliances are installed or which have an attached garage must install CO alarms in the general vicinity of each sleeping area. (Effective with adoption of the 2009 IRC)
NE Revised Statutes 76-606:
The owner of a single-family dwelling or a dwelling unit in a multifamily dwelling that is used for rental purposes shall ensure that an operational carbon monoxide alarm is installed on each habitable floor of the dwelling or dwelling unit
Requires the seller of each existing single family dwelling or multi-family dwelling offered for sale or transfer on or after January 1, 2017 that has a fuel fired heater or appliance, a fire place or an attached garage to have an operational carbon monoxide alarm. Signed into law March 2015.