Floor plan design single-family house solid wood construction 140 sqm in Lower Saxony

In my eyes, an island only really comes into its own if you have a completely open kitchen area. The island then separates the living and cooking areas from each other. If the kitchen is a separate room with a passageway, no matter how wide it may be... an island always looks forced and not well executed...

Because through the (narrow) passage, the island is no longer a room divider, but an obstacle right behind the passage.

An island, in my opinion, only really comes into its own when you have a completely open kitchen area. The island then separates the living and cooking areas from each other. If the kitchen is a separate room with a passage, no matter how wide it may be… an island always looks like it was forced rather than natural…

Okay, that makes sense. :) We can't avoid the intersection points, but the walls, which according to the current plan have been pushed further out, could definitely be shortened as much as possible to make the kitchen itself more open – do you think it's worth pursuing this idea further? Or would it then be neither fish nor fowl?

Or is a rough hand-drawn sketch enough for now, showing how we roughly imagine it?

Definitely. In the end, you’ll do yourself a big favor if you deal intensively with the measurements. Draw, measure, draw, measure…

More than a small light bulb was definitely planned there.

Regarding the lighting situation, we would install relatively densely spaced LED spots in the ceiling for the basic lighting

also some wall lighting here and there, accompanied by indirect light sources for an evening cozy atmosphere.

Artificial light can never replace daylight. No one wants to keep switching lights on somewhere constantly. And it’s not about the willingness to switch but about the dependence on that little lamp up there, whether LED or more than a light bulb. Light can also be overwhelming.
And who wants a hallway stub that is the route to the emergency toilet and sinks into darkness? Plus the fear of stumbling over a shoe… a window belongs there.
… and you asked about window sizes: at least 10% of the living area is necessary. You have that.
Quite a few people have 30%.

The windows are actually currently more symmetrical than sensible...

I see no symmetry!

Then I read somewhere that the planning absolutely has to start in the upper floor, i.e., I recommend throwing everything overboard and first carefully questioning and writing down the room program.

Yes, that was suggested once by . I now also do this with terraced/semi-detached houses and stair problems with sloping ceilings – a very helpful tip! :)

How can I determine the exact heights of the individual reference points here, so that we are not left in the dark and can plan precisely?

Try calling the municipality. They should have plans for the development of the building area, and if they impose regulations on you through the zoning plan that depend on that, they should take care of informing you about the reference point. As for the kitchen ... yes, you can outsource a lot if you want. It's not comfortable. But on 3 meters, you only get about one good meter of island. That's meager. With the peninsula, you get 2 meters plus 1 meter passage, but that’s not nice either. So better to put everything you need in the kitchen directly in the kitchen.

Why is everyone talking about an island?
I don’t read anything about a desire for an island in the initial post.

open or closed architecture Living/dining/cooking area preferably open
conservative or modern construction conservative
open kitchen, kitchen island open kitchen yes, kitchen island currently not planned

Can we basically (without detailed planning) sensibly plan a kitchen (surrounding as a U) in the existing space, or do we have to redesign the walls at the bottom beforehand?

If an island is desired, this is the wrong floor plan.

I am an electrical engineer, not a designer, and I am just sharing my personal perception here ;)

But I think as long as only the passage becomes wider and the dining/cooking area remains a 3m wide "tube," a kitchen island will not "tell" what you expect from it.

But it doesn't necessarily have to be a kitchen island either. Also keep in mind that a U-shaped solution with all cabinets against the wall is significantly cheaper than kitchen cabinets standing freely in the room or an island.