for Perfect Fit Surfaces
Opening a drawer and the drawer bumps into the knob of another drawer.
There should have been a filler in the corner. A filler is a matchng piece of wood that provides sufficient space in the corners.
When the walls are not square and straight, the counter tops will have gaps among other problems. the walls need to be squared and true before installation. Cabinets may not fit tight against the wall and the countertop might have gaps and spaces.
The new drywall needs to be removed and reinstalled with straight surfaces resulting in a surface that is straight and true.
Uneven floor means that all the cabinets must be leved from the highest point of the floor, to keep surfaces perfectly flushed and leveled.
First thing to address is the electrical in kitchen or bath. The problem is electrical codes call for minimal number of outlets required. But there may not be outlets where you need one. Chances are your new design will differ from your old kitchen.
This is the opportunity to get tyour electrical the way you need it for your new kitchen or bath. This might include having a sufficient number of receptacles in the right locations. You might consider having added electrical wiring for new lighting above the cabinets, below, or even in the toe space. There are many options for many types of lighting and design to work smartly with cabinet and workspace layout. Your electrical plans should include a space for charging your phones, for example. In any case, thought and planning should be included for conveniences and kitchen use efficiency.