What is the green circle next to the hammer in the building up grade?

In order to upgrade your buildings, you must collect all the necessary materials listed before you can start construction. A green circled check mark will appear next to the materials when you have enough. Each building has a total of 5 upgrades that require materials. After the 5th upgrade, you can buy the Golden Façade upgrade with gold bars, which includes additional flower and the ability to select any of the 5 previous models. You can also use gold bars to buy a particular model, but it is a bad deal.