The interior of Berk's Storehouse appears to have an open concept - there aren't separate rooms. There are pillars supporting the roof that have artwork on them, though these were destroyed during How to Start a Dragon Academy" when the dragons raided it and burnt it down.
The Berk Storehouse appears to be situated on the left side of the Great Hall and partially overhanging the cliff. It is approximately where the future Dragon Hangar will be. There is a functional windmill at the top, though its purpose is unknown. There are a set of large double doors facing the village.
The Berk Storehouse is a large storage space for food, for all the villagers. It gets stocked during the warm months and sustains the Tribe during the cold months.
In "How to Start a Dragon Academy", Stoick oversees the stocking of the Storehouse, in preparation for the upcoming 'freeze'. However, the resident dragons have not been trained, and raid the storehouse and set fire to it. The building is severely damaged.
During the episode "Viking for Hire", the Berk Storehouse appears in the background, indicating it was rebuilt with the same design.
In "Animal House", Stoick and Gobber check the supplies in the Storehouse before an unusually early winter storm approaches. They don't nearly have enough stock, and the dragons are inadvertently to blame, because they stress out the farm animals, causing them to not produce eggs or milk.