Let's say we have a room of 20 m². Now plan a window with a glazing area of 2 m² for this room.
How bright is it? How bright is the room with 10 m² of glass? How much m² of window is unlikely to make the room brighter?
Is there a rule of thumb with which to figure this out, or perhaps even a calculator or small formula to figure it out?
Maybe even with outside influences such as a nearby tree, carport, neighboring house, balcony over it, north/east/south/west orientation of the window?
According to state building codes in Bavaria, about 1/10th of the area of the room should be windows.
The tl value tells you how much light passes through the window.
Now you could theoretically plan for 20 m2 of window area, so you would have tl70% light transmission.
But you never need that much, or don't want to put furniture in?
As an example, the number of lux indoors cannot be achieved even remotely by evenly distributed sunlight.
It also varies between cloudy and sunny, winter and summer.
It is not just the net area of the window that determines the lightness of the room, but also the height of the lintel. There are several rules, for example, the depth of the room should not exceed twice the height of the lintel.
In addition, different rooms have different requirements.
Our HWR room has enough light at 2 sq. m. by 8 sq. m., our living room has 14 sq. m. of windows by 25 sq. m. – It's nicely light. The children's rooms, with 3 sq. ft. windows per 16 sq. ft. space, are also pleasantly light. The hallway is ~2 sq m, which in cloudy weather is sometimes a little dark.
I would take as much glass into the house as possible. Glass (in a plastic frame) doesn't cost the earth and reduces masonry costs if the glass is floor to ceiling. Bright rooms are eye-catching, and I haven't heard of rooms being too light at the end of construction -> at least if shades have been put on them.
Lux decreases every meter deeper into the room. There are calculations for that.
But the calculation for a fictitious light for a fictitious room would be very poor and result in rough approximate values.
If your room is two meters wide and 10 meters long, and it is dark at the other end of the room, the window should be placed on the narrow side.
However, the architect should be familiar with this.