A communal block of flats might often have an outbuilding or garage area where residents and leaseholders can store items, and where larger items such as lawnmowers might be kept for gardeners. These outbuildings can sometimes be a security concern if they are easily accessible and in a vulnerable location.
Outbuildings are best protected with a variety of security measures, including a high quality door (something like an up and over garage door or a well fitting roller shutter can work very well), an alarm system, and a CCTV system as well. These measures will show the property is secured to act as a deterrence measure, as well as putting up boundaries between valuable items and potential intruders. Alarm systems will come with motion detectors to help keep garages and outbuildings protected if an intruder does gain access too, sounding an alarm that will alert nearby residents to the break in.