How much light does a window let in?

Most people don't know what to do with lux... just as a side note. Moreover, a lux value in a room cannot even remotely be evenly distributed by sunlight. It also varies between cloudy and sunny and winter and summer. Not only the pure window area determines the room lighting, but also the height of the lintel. There are quite a few rules of thumb... for example, that the room depth should not exceed twice the lintel height. Different rooms also have different requirements. Our utility room has sufficient lighting with 2 sqm of windows for an 8 sqm size, our living room has 14 sqm of windows for 25 sqm - that is pleasantly bright. The children's rooms have 3 sqm of windows for 16 sqm of room, also pleasantly bright. The hallway has covered ~2 sqm, which is sometimes a bit dark in cloudy weather.

I would take as much glass on the house as possible. Glass (in the plastic frame) doesn't cost the world and reduces the mason's costs if the panes are floor-to-ceiling. Bright rooms are inviting, and I haven't heard that rooms at the end of the build were too bright -> at least when shading was added. Fortunately, my wife prevailed with the windows :)

The lux level decreases with every meter into the room. There are calculations for this. However, it will be very difficult to calculate for fictitious light in a fictitious room and will lead to rough guidelines. If your room is two meters wide and 10 meters long, it will be dark at the other end if you position your window on the narrow side. An architect should be familiar with this :)

Landesbauverordnung sagt in bw muss ca 1/10 der raumfläche, fenster flächen sein.

No - 1/8 according to DIN 5034

Rhineland greetings

Nope - ⅛ according to DIN 5034

Rhenish greetings

Nope, in Baden-Württemberg according to the state building authority 1/10, just checked again. Other federal states have 1/8

Other federal states have 1/8

And where is the OP building ...? Rhineland greetings