Buildings define the core functionality of your base. Each Building Group corresponds to a unique function with its own separate bonuses that compliment the overall experience. They also have levels that unlock new features and content. To level up a Building select it and choose Upgrade - make sure that you have sufficient resources and meet all additional building-related requirements. Buildings that meet the construction requirements for an upgrade will appear with a green marker on their level label (on the left corner).
Every Building Group has it's own set of Facilities, and each one has specific bonuses and rewards in terms of Force. All buildings have a construction cost in one of more resources and also take a certain amount of time to complete. When starting an improvement you occupy one Construction Worker from the Build Queue. There is only one such slots at the beginning of the game but you can open a second slot by using a corresponding item. You can also finish up a Building construction or upgrade for free and the time for that depends on your current active VIP level. Once Building Construction or Upgrade is complete, the corresponding bonuses are automatically applied to your City.
The same construction rules apply also when building your Headquarters or any of the available Add-Ons