QUESTION: "Can I add a deadlock to my apartment door?"
The short answer: YES!
The Slightly Longer Answer: Yes, but... the lock needs to comply with building code regulations so you can't install just anything; it needs to comply with the laws around fire containment.
Recently, the N.C.C. (National Construction Code) regulators along with the Master Locksmiths Association have confirmed that the Lockwood 001 / 002 style dead-latches are allowed to be installed on unit entry doors if the hole drilled is <32mm on a compliant door. These locks are self-latching so the door will always close to contain the spread of fire. It must use the correct strike (e.g. a metal frame strike for a metal frame door) and the door cannot be otherwise modified for the installation... I've seen chunks of the door removed to incorporate the wrong type of strike, which is an expensive mistake.
Other types of locks such as deadbolts are NOT allowed as they have the ability to block the door from fully shutting which allows the fire to spread. It's a strange regulation, as the bolt would have to be thrown to interfere, but it COULD happen so the probability has to be taken into account.
Electronic locks should also be checked for their fire-rating before installing as a primary lock let alone installed as a secondary lock. With the rise of short-stay rentals in multi-dwelling buildings the use of cheap, non-compliant locks is rife. Just because you've seen one installed on someone else's apartment, it doesn't make it legal.
Another consideration for fire door work is if the door contains asbestos. Asbestos was used as a fire retardant in apartment doors up until the late 80s. Your strata management company should have an asbestos register and hopefully documentation on the doors. There should also be a metal tag on the door/frame which shows information about the door... it might be illegally painted over. This can help identify the year of manufacture.
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