This is an example of flooring construction. As a house ages, the flooring will begin to deteriorate and sink in places. In most homes, the subfloor joists are solid. In such cases, new flooring will be laid on top of the existing flooring. This time, the kitchen flooring work included an underfloor storage unit, and new flooring will be processed and installed to fit the lid. It's not just the lid that will remain old.