Immediate thought is do you own the flat or are you renting? That might affect how far you can go.
I’m wondering if the buzzer would be enough to trigger a vibration sensor? The ST multi-sensor has vibration I think.
Alternatively you might be able to modify a contact sensor so that the buzzer closes the contact. See this thread for further ideas: Z-Wave Doorbell from a Wireless Doorbell + Door/Window Sensor
I’ll tag @JDRoberts but he’s probably already working on a response as I type 