Land expansions unlock at level 20 and will become available on the land surrounding your restaurant as you level up.
You will see a wooden sign on the land with a lock around it and a number telling you which level the land will unlock at.
Once an expansion is available, you will see a green check mark on the sign and when you tap to open the expansion menu, a list of requirements will appear.
After you have collected or purchased all of the needed materials, click on the green Expand button at the bottom of the screen to complete your expansion.
Once your expansion is complete you will need to clear the land of any obstacles such as tree roots and boulders before you can build on it.