A method and a system for emergency evacuation of building occupants and to a method for modernization of an existing building with the evacuation system. The system is used in a multi-story building having a plurality of floors and with at least one elevator positionable at selected landings of the floors. A first device is provided for measuring the number of persons in the building and a second device is provided for detecting an emergency condition in the building. At least one control unit is provided for determining or for estimating a number of building occupants in the building. The control unit defines at least one evacuation zone in the building during the emergency condition. Based on this information, the control unit defines at least one designated floor in the building during the emergency condition. Then the system evacuates the building occupants with the elevator car and/or a stairway from the evacuation zone to the designated floor.