Base kitchen cabinets should sit directly on the floor. They are designed to be freestanding and do not require any additional support or elevation from other cabinets or DIY structures. Placing base cabinets on top of other cabinets or DIY structures could compromise their stability and may lead to safety issues, especially if they are not properly secured to the floor.
During the installation of base kitchen cabinets, it's essential to ensure that they are level and properly aligned. Shims may be used under the cabinets' feet or along the base to level them if the floor is uneven. Once the cabinets are leveled, they should be securely anchored to the wall studs and possibly to the floor to prevent any movement.
For the best results and to ensure a safe and functional kitchen, it's recommended to follow the manufacturer's installation guidelines or hire a professional cabinet installer who has experience in setting up base cabinets correctly. This will help you avoid any potential problems and ensure that your kitchen cabinets are installed to the highest standards.