Building complex is a way to have several stations of any type together in one place. For instance, you can combine fire stations with rescue and police stations. This is especially applicable when in reality these buildings are not separated. Creating building complexes will make your map less crowded and allow it to provide a better overview.
 
How do I create a building complex? 
In order to create a building complex, you will need to create a building complex base first. Select a station that you would like to upgrade to a building complex base and go to the new tab in it called “Building Complex”.
 
 
Once the process of base creation is finished, you will be able to add 4 more stations to the building complex in a range of 10km around the complex base. 
 

Once you attached 4 stations you will have the option to build the building complex which will complete the process.
 
 
All the stations will combine into 1 icon on the map (other icons will vanish).
 
 
 
What is the difference between a small building complex and a regular building complex?
 
Small building complexes consist of 3 small stations: The base and two additional stations. Regular building complexes consist of 5 normal stations: The base station and 4 additional stations. Small building complexes provide fewer upgrade levels. If you have started upgrading one of your small stations to a normal one, it can't be used for a small building complex. Both small and regular building complexes can only built from stations that are be at their maximum level.
 
What advantages does a building complex have?
 
Creating building complexes will make your map less crowded and allow it to provide a better overview. This will make it easier for you to manage your missions. You can upgrade your building complex to give you more vehicle spaces and better vehicle efficiency.
 
Please note that the building complex does not have any extra influence on the missions that are generated and the buildings are still counting separately for mission requirements etc.