Stone Storage can be found on Home Island. It will unlock at level 1 when you complete the quests "Craft fiber" and "Get the Forge from the Shop".
Upgrading your Stone Storage will increase the number of slots you have for storage of all items.
It can be upgraded 9 times throughout the game, with each upgrade requiring a different set of items.
For more info on upgrading your Stone Storage, check the guides we have posted to our official Facebook group and Discord channel.
To learn more about building, check out the Crafting Stations section!