Usually code requires these structures to have a fire wall as the common wall and it reaches up through the attic to a common roof.

It's just a firewall to seperate (significant) chunks of net worth, nothing more. 

I'm sure automobile manufacturers have somehow broke the code and are able to lease their vehicles without being liable for any boneheaded stunts the driver pulls, but I'm not so sure that kind of "firewall" can be constructed between a piece of real estate and the owner on title.Interesting thought, however.

In Arlington TX we have to 1) install fire suppressant systems in each unit (attic suppressant systems in attics), 2) you will also need a "firewall", we use a 1/2 DensGlass application, 3) fire rated decking within four foot of "firewalls", 4)and non-vented soffit within four foot "firewalls."

Now having said all that, if my STR was attached to my house where I lived, I would absolutely have firewall separation where there would be no way they could reach my house network from the network at the STR.  

Next time, we will go with wood fire walls on the second floors.

There are building codes you must meet, you must meet snow loads, wind loads, egress, potential fire suppression, site drainage, parking, there are minimum and maximum height allowances, anything above 3 stories must have fire suppression, anything over 12,000 sq/ft must have fire suppression, if the units touch each other they must be fire blocked, you need 3hr firewalls between each x amount of sq/ft.

Storm ponds, storm sewer, property water flow, fire hydrants, utility access, sidewalks, soil compaction, driveway access, landscaping, facade requirements, fire suppression, fire walls or building separation distance, fencing, setbacks, required parking, outside lighting, epa, Corp engineers, grading/elevation, etc.  

Done correctly it is basically a legal fire wall between your businesses and keeps the fire to spread from one to the other. 

Just exterior walls and fire wall plan for something already constructed.