You may need a Warehouse as temporary storage for buildings. Choose a building, tap the Move button at the bottom of the screen, and then choose the To the Warehouse option. The Warehouse icon can be found in the lower right corner of the screen.

You can get your building back whenever you want. Buildings in the Warehouse require neither power nor staff, but you don’t get any profit from them either.

If you are out of storage space and some buildings still need to be removed from your Island, you can always buy a cell, which will remain in the game forever and will definitely come in handy in the future.

If you purchase a building or get it as a reward, it goes to the warehouse automatically, but doesn't take or add a real slot. Such buildings are marked with a feather sign.
The Warehouse has 2 more tabs along with the one with buildings. The Items tab contains accessories, which can influence buildings or island parameters. The rest can be found in the Resources tab.