"The code" you need to be looking at is most likely your local building code. Here in New Jersey we use our own version of the International Building Code (IBC), 2009. It lists the various occupancies and what, if anything, is required in the way of a fire alarm. It's amazing how little is required.
Whether or not the property is private or not would not drive the requirements in NJ. It's all about the occupancy, or use group.
On the other hand, NJ has what's called the "Rehab Code" and it strictly limits what can be required of a property owner when new work is performed in an existing structure. As a rule of thumb, unless you are affecting more than 50% of the building's square footage, you can't be required to bring it up to current code. So a use group that didn't require automatic detection previously still doesn't even if a new building of the same use group does. Again, it's a rule of thumb so please pick no nits.