2010 ADA Standards for Accessible Design Pocket Guide
809.5.5 Residential Dwelling Unit Primary Entrance.
Communication features shall be provided at the residential dwelling unit primary entrance complying with 809.5.5.
A hard-wired electric doorbell shall be provided. A button or switch shall be provided outside the residential dwelling unit primary entrance. Activation of the button or switch shall initiate an audible tone and visible signal within the residential dwelling unit. Where visible doorbell signals are located in sleeping areas, they shall have controls to deactivate the signal.
A means for visually identifying a visitor without opening the residential dwelling unit entry door shall be provided and shall allow for a minimum 180 degree range of view.