Lighting and electrical planning in new construction

.... normally one ceiling outlet per room is sufficient :rolleyes:

Depending on the room size, that can be too little. Several diffuse lights or sometimes an additional wall lamp do many rooms a great deal of good.

Our choice fell on dimmable LED panel lights, adjustable from 3000-6000K, and lamps with E27 sockets,

Be careful with dimmable lamps, some flicker like a cow’s tail... And those remote controls for every lamp are also annoying. High-quality flicker-free lamps are recommended at least in living areas. And in workspaces, flicker-free daylight lamps with a high CRI are advisable. There are some standards whose compliance is a good indicator. But you can only find those in specialist office trade.

And in general: forget about spotlights and rather rely on diffuse, broad lighting. Spots can, however, be used as an additional effect in special places.

I need to take a closer look at the access points first. I don’t even know what they look like. On the ground floor, we want a concrete ceiling. I would like to drill into it as minimally as possible.

For me, these are wall outlets directly under the ceiling. On each floor in a central location, e.g. in the hallway. An access point with PoE fits perfectly there; Ubiquiti was already mentioned as an example. Especially with concrete ceilings, one access point per level is a must. The concrete ceiling attenuates very strongly.

I was advised by many to not save on power outlets as much as possible. Mainly to retain maximum flexibility, not necessarily to occupy them all permanently

That is true, but ultimately these well-meaning tips exist for all areas, basically to do more and more of them. I currently look around in my own place and try to see what my "power outlet behavior" is and then maybe add one or two more; but I don’t want to plaster the whole thing with them. I like spots less, even if or maybe because they are currently so fashionable. They don’t work for me as pleasant general lighting; for accent lighting for pictures etc., I would prefer to be a bit more flexible, for example through a track system if needed; otherwise, I find wall lighting a nice option. You also have to switch the whole thing and from different points each time, and for me, too many switches or switching options would almost be annoying. That doesn’t solve your "problem" now, but I thought it more as a counterbalance to the "too much" out of concern to not have too little.

Above all to maintain maximum flexibility, not necessarily to occupy all permanently. Are there average values/recommendations regarding the number and living area?

Well, flexibility is all well and good, but I would still roughly consider where the number of sockets makes sense for you. I think 15 sockets in 15 m² in the children's rooms is excessive. Half of that is more than enough. At key points where you might need sockets in 10-15 years by rearranging furniture, you can also have a flush-mounted empty box plastered in and just lay one cable in. If you ever really need sockets there, they can be very easily retrofitted. For me, all the sockets behind cabinets and beds would be dead capital because they are paid for but unusable.

You have to decide the number of sockets. In our house (about 145 m²) we get by with around 85 sockets, others with significantly fewer or more.

Depending on the size of the room, that may be too little. Several broad-area lights or sometimes an additional wall lamp do many rooms very, very good.

For me, these are wall outlets directly under the ceiling. On each floor at a central location, e.g. in the hallway.

That's exactly what I mean. In rooms of 15 sqm, that is enough. About sockets: making some switchable here and there and possibly installing a two-way switch (e.g. by the bed) makes sense. 15 for children's rooms is exaggerated. Just try playing through the devices and daily life.

You have to decide the number of sockets. For our house (around 145 m²), we manage with about 85 sockets, others with significantly fewer or more.

We have 149 including basement and garage, and although we have gone through everything hundreds of times, since moving in I have noticed 3 spots where an additional socket would have proven practical. Certainly not a drama, but what our electrician said at the beginning has proven true: "You can never have too many sockets." Pre-installing sockets seems nonsense to me; it costs just as much as a finished socket.