Hi Chef,
As you progress through the game you will need more space in your Restaurant to place Tables and Appliances.

At level 6 and above, you can purchase plots next to your Restaurant. Click on the empty plot to see the Coins and Tools required to purchase the plot. If you have the required items and Coins you can purchase them by clicking on the Expand button.
Additionally, you can also use Expansion Assists if you are short of Tools for expansion.
Here's an image for your reference: