What should I watch out for when installing lighting in my flat?
If you hired a licensed electrician then he should be aware of any building codes in your area. I am trying to understand your question though. You stated "I've decided to add a layer at the ceiling and rig a bunch of spotlights there." Can you clarify this statement?
The only thing that brings up a red flag is how many amps are going to be drawn by the spotlights and how many other fixtures and outlets are on the same circuit? Is it a 15-amp circuit or a 20-amp circuit?

As a rule of thumb in counting devices on a circuit you count each outlet (light or receptacle) as 1.5 amps. Now remember you only count 80% of the rating which means a 15 amp breaker should not exceed 12 amp load, a 20 amp breaker would be 16 amps.

Check out this previous post. How many Outlets can you put on a 14-2