The mezzanine level can be raised to whatever the height allows and suits the area - sometimes a couple of steps is enough to make a difference visually; sometimes you might need a short flight of stairs, perhaps curving around a corner. Stairs are expensive to install so I would generally avoid the latest fashion and go instead for classic good looks that will stand the test of time. In a kitchen, safety is an issue, so choose a slip-free surface with low risers and deep treads. The key to a successful split-level kitchen is to make the two zones work well together. It doesn't particularly matter which way round you arrange the kitchen and diner but you want to avoid a top-heavy look, so don't put floor-to-ceiling units on the upper area. Bear in mind, too, that you mostly stand in the kitchen, and sit in the dining area, so if you want to chat to friends while cooking, the kitchen is better on the lower level. Heat rises, so this arrangement will also be more energy-efficient. It's a good idea to try and minimise household traffic through the dining area to make it a quiet and cosy spot.
The biggest problem with split-level kitchens is a lack of worktops and storage in the kitchen area. There is simply less wall space. Most split-levels have a mid-height structure between the two zones which you can utilise on the kitchen side by running work surfaces and below-waist storage along it. Pull-out storage or even larder space can be added under the mezzanine provided you have safe side access. Noise and smells float through split-level kitchens, so use soft furnishings to absorb sound and install good extraction. I would always make sure that lighting for the two areas is independent and can be controlled from both lower and upper floors.
