Begin by selecting the cabinet that you want to change the countertop thickness of.

Under "Cabinet" Menu, look for the section labeled "Countertop" on the right of the design panel. It will be available both in the "Quick Access" and "Customise" tabs.

Click on the drop down next to it to open the details of the countertop. You will see the "Extension options" section there. Click it to open the drop-down menu showing the details for this.

You will be four sections - Left/ Right/ Front/ Back where you can input a numerical value in mm to create an extension on that side of the cabinet.