Building upgrades provide permanent bonuses to each production line within the chain. For example, upgrading Greenhouse Speed increases the speed of all production lines inside of the Greenhouse.

Building upgrades are Epic (pink) rarity cards and are guaranteed to drop in Epic, Legendary and Diamond chests.

You can see the accumulated stat bonus provided by all your building upgrades for each building toward the bottom of the building upgrade window. These stats multiply or add to the stats of each individual production line within that building.