Add-ons are Terran buildings that provide additional functions to attached unit producing structures (Barracks, Factories or Starports). The two add-ons currently in the game are the Tech Lab, unlocking production of advanced units in the attached building and offering unit upgrades, and the Reactor, allowing the attached building to produce two units at the same time.
Unlike most buildings add-ons are not built by workers but by Barracks, Factories and Starports. As add-ons always need to be located at the bottom right of the production building to be properly attached, every building can only have one add-on and every add-on can only be attached to one building at a time.
When a building with an attached add-on lifts off, the add-on becomes inactive, in which state it does not provide any services to buildings and is not capable of research. Landing a compatible building in the correct spot re-activates the add-on and attaches it to the landed building. This facilitates tech switching and allows for maneuvers like building a Tech Lab with a Factory while a Starport is in construction and then landing the Starport next to the Tech Lab as soon as it is complete to reduce the time it takes to get a Banshee out.
- When there is ongoing research in a Tech Lab when the attached building lifts, that research is canceled.
- When an add-on is destroyed currently building units in the attached building are not affected and will complete as if the add-on was present.
- When you attach a Barracks, Factory or Starport to an inactive add-on of another player, that add-on's ownership changes to you.
- In the Wings of Liberty Campaign, a third type of add-on exists - the Tech Reactor. This add-on has the advantages of both a Tech Lab and a Reactor, with no drawbacks.