On each level of your home
Outside and near sleeping areas such as bedrooms
In the area of any major gas-burning appliances such as a furnace or water heater
At least 15 feet from gas-burning appliances (not any closer)
Up high, since carbon monoxide may be contained in warm air coming from your appliances

In general, follow the manufacturer's instructions.

Scroll through the floor plan renderings below for examples of CO alarm placement.

One Bedroom unit (not to scale):

One bedroom rendering

Two Bedroom Unit (not to scale):

Two bedroom unit rendering

Two Bedroom Unit with Gas Dryer (not to scale):
Note: If bedroom doors are more than 30 feet apart, two carbon monoxide alarms are required.

Two bedroom unit with gas dryer

Two-story Two Bedroom Unit (not to scale):

Two-story two bedroom unit

Three Bedroom Unit with Gas Boiler In Basement (not to scale):

Three bedroom unit with gas boiler in basement

Three Bedroom Unit with Gas Stove and Fireplace (not to scale):

Three bedroom unit with gas stove and fireplace

Efficiency/Studio (not to scale):

Efficiency/studio unit

Unit with One Bedroom That Has Direct Access to the Kitchen, with Gas Furnace In Basement (not to scale): 

One-bedroom unit with access to kitchen and gas furnace in basement

Apartment Containing Gas Fireplace (not to scale):

Apartment containing gas fireplace

Cross-Section Showing CO Alarm Placement (not to scale):

Cross-section of home showing CO alarm placement