This apartment has a big open space that have to be defined and to solve this task I was using concept of loft design. That means that space is modern, with different kind of materials and textures. The structure and color of that material have important role giving very individual character of place. You can see on the technical plan two new walls and some corrections with one column and the door of guest toilet. Displacement of this door will enable better organization in the toilet (first department is for washing machine and some kind of larder and the second one is for a little toilet) and free wall for shelves in working zone.
Living zone has very good south light which is great for winter, but problematic summer. My proposal is to cover big windows with white rolo courtains. Floor is covered with light lamett (you can see my proposal for the nuance). The walls have different treatments. Some are colored with light grey -blue ton, one is red to activate the energy in the communication zone and we have also some red details for the same purpose. The structure on the north wall can be made with light stone or wallpaper. This zone is divided on four sections: sitting (TV set, library, coffee table), dinning, working (writing, playing piano) and entrance area. Furniture has simple modern lines (some pieces are hand-made) but to make the original I put several designed pieces (lamps, commode, sculptures, paint and off course live plants to revive the place). For the nice atmosphere are responsible the lights of the suspended ceiling and several lamps on different positions.
Kitchen has added elements on the wall and new tiles. I think that it would be better if you imbed the fridge in the rest of furniture.
Bedroom has also a little of red which is good for young people. I have a hand-made proposal for walk in closet but I think it would be better to use one modern fitting cabinet. You can put a personal training equipment behind the door. Bathroom has very nice and elegant tiles (Casa dolce casa). Between the room and the bathroom is a wall with one glass part (glass cubes) to allow more light comes in.
These are the basic information.
Good luck in your arrangement.