hey rambola! i personally love sculpting these little spaces, where every detail counts. i dont know how much freedom you have when designing, I recommend going wild and investigating how you can stretch those 38 square metres.
here's a similar-sized flat I had to design recently, hope it helps to get those designing juices flowing!!

section through the flat. i had to hide the wardrobe, drawers, etc, even the staircase was foldable!

temp2.jpg

If you study the figure/model on which you drew the red things, you can learn how to do what you want.
Start with the first notes at the bottom of the model.
For instance:
start with a circle on the ground, then use pushpull tool to make a cylinder.
Select the top face, then scale face larger or smaller than the original diameter.
This model is an excellent way to learn these techniques.

EDIT: ok I see that others have posted.