Here are some nice examples showing of how you can integrate a mezzanine into your flat.

Use a spiral staircase to save space. Metal balustrade and the staircase add a touch of raw industrial beauty.

Make a feature out of it. When you can only add a small mezzanine, try to make it look special, as a great decorative addition to the existing space, rather than just an addition, that perhaps visually can even make the room look smaller.

Add a window. If there is no chance to add an actual window to the outside, add an interior window to let it some daylight.

Make use of the staircase. The access to the mezzanine will take some space, so why not to use it smartly. Staircase could be transformed into a joinery piece or extra storage.