On the right hand of your Restaurant you have the possibility to buy additional land slots in order to obtain more space to place more tables to cater to more customers. This addition to the game will be unlocked once you reach level 6.  You can choose the area you wish to expand and tap on it. 
Coins are required for Restaurant expansion.

For more detailed information, please scroll down to the image below to see where the new plots can be purchased for expansion in Star Chef.