There are two different Warehouses on Family Farm Seaside - one on the Seaside Farm and one on the Island Farm.

Both Warehouses can be used to store items such as Animals, Machines, Decorations, Buildings and Collectable items but the Seaside Warehouse can only store Seaside Farm items and Island Warehouse can only store Island Farm items.

Here is what the Warehouse looks like on the Seaside Farm (left) and on the Island Farm (right)

 

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The Seaside Warehouse is unlocked in the Building Area plot #2. 

The Island Warehouse is unlocked when you unlock Plot 10.

Both warehouses can be leveled up to store more items!