You can often see this button when you open the manager's menu in the village.

It happens even if you have already built the house, but what does it mean?

That means you don't have a house owner yet.

After you receive the homeowner's card, it will be possible to upgrade it to increase the collection speed or house income.

You can receive the house owner cards by purchasing a house owner chest in the Village shop.

Or by participating in the Green Island event.

Happy hunting! :)