Prerequisites: Main Building Level 1
Crop produced by your croplands is stored in the granary. By increasing its level, you increase the granary’s capacity.
After building up one granary to level 20, additional granaries can be built.
The level dependent attributes (such as construction costs, generated culture points and capacity) can be found here.
For the full table of construction times click this link for server with 1x speed and this link for server with 3x speed.