How many windows should one have?

oriented towards the state building code

oriented towards DIN EN 17037, which gives 20-25% as a rule of thumb

But these are only guidelines!
And they apply to living rooms. Although a bedroom is also a living room, if you only use this room for sleeping, then a 15 sqm bedroom with an average window size is sufficient. In contrast, a multi-purpose/living room can benefit from much more window area if you consider that many activities take place at the dining and living room table. Also, a good distribution with knowledge of the sun’s position in the north and south is important. Light serves well-being and good visibility.

Of course, I could also just take the standard floor plan from the house builder, then 10% according to the state building code is met everywhere and upstairs and downstairs each room only has windows facing one direction :) But I don’t want to live in something like that.

Then you have to hire an architect who will build you a light-flooded house.

So now we ask ourselves

Why ask when it comes to common sense?
Where there is a window, you don’t need artificial light during the day. A storage room can manage without a window. But if you use the room 10 times, then at least a small window is advisable so the light switch doesn’t wear out. In the kitchen, daylight is nice where you probably stand 10 times a day to do or prepare something. In children’s rooms, two windows on two walls would be optimum, because they can be lit from different sides over the room for many different activities, but often this is not possible.

I would never measure the glass with a tape measure. The window itself is the guideline.
And if you plan the floor plan with common sense or have it planned, then you follow the daily routine. Everyone knows that you feel more comfortable in a room with daylight than in a room with a small opening. Each of us has already lived in apartments or houses and knows windows and light incidence.
And as I understand it, this is about a single-family house of normal size without extra wings or studio, not a status object where the house is meant to blend with the forest or similar.
If you have a floor plan, you can ask here in the appropriate subforums – having some sense of design for exterior façades and partition walls or not should also be taken into account.

If there is a need for discussion, the best thing to do is to sit down with the architect and talk about whether window X can still be enlarged or if another window can fit in spot Y. He can answer that pretty well by feel. An experienced architect also pays attention to ensure that the windows on the different floors do not look jumbled together but rather appear harmonious.

Well, this is probably not about individual planning, but about the fear of not recognizing a "lower quality" general contractor proposal:

Of course, I can also take the standard floor plan from the house builder, and then just meet 10% of the state building code everywhere [...] If he wants to sell us only the minimum state building code to save money [...]

The notion of the house builder making his margin by leaving out window area is unrealistic. "Window area according to DIN" or "light on the construction side" I have never read in any construction service description, not even at discount providers.

Unfortunately, I couldn't find anything on the internet about this, hence the question here.

If on a topic you find nothing but yawning emptiness even in the boundless internet, either you’re too dumb for the right search terms or (more likely here) the only freak interested in it. Not a single house provider advertises its standard over-fulfillment level regarding window areas (I want to push it in Rhineland ultimate maximal terms: "not a single one at all")!

Of course I can also take the standard floor plan from the house builder, then just 10% of the state building regulations are met everywhere and on the upper and ground floors there are only windows facing one direction per room :) But I don't want to live in something like that. It's not about us planning everything ourselves but about being able to speak the language, to participate in a layman's way, and possibly to bring in special requests like more light. If he wants to sell us only the minimum state building regulations to save money but double that would definitely be better and we could save money elsewhere, then I want to be able to bring that up.

Good grief. Then plan 2 windows per room? But keep in mind that windows also strongly influence how you can furnish a room sensibly. By the way, light can also be disturbing... when watching TV, working at the computer, sitting at the table... In my study, I have a window facing south, I sit perpendicular to the light incidence. Do you think it would make sense to have another window "behind" my desk to the west? Probably not, right? ;) And in the living room, on the south wall where the TV hangs, I also don't have a window :(

If you want to have a say in this, you have to work your way into the topic. Buy the standard and get started. No one can take that off you over a few lines in a forum.

But to approach something like this only via a minimum requirement of the state building regulations... are you an engineer at Audi or Siemens? ;)

You are overthinking it. Even a layperson has a sense of whether a room will be bright or dark. If there is a need for discussion, the best thing is to sit down with the architect and talk about whether window X can still be enlarged or if a window can be added at position Y. He can answer that quite well based on feeling. An experienced architect also makes sure that the windows on the different floors don’t look thrown together but rather harmonious.
Basically, you have a small disadvantage in a one-and-a-half-story house. And if you want a closed kitchen, too.

Thanks, that helps, for the moment we will plan some currently planned windows slightly larger and reserve money for possible additional windows that we currently see as a nice variant so that we have enough leeway in the architect discussion.

But those are only guidelines!
And they apply to living rooms. Although a bedroom is also a living room, if you only use this room for sleeping, then a 15 sqm bedroom with an average window size is sufficient.

Thanks for this as well, such an assessment helps us to better classify things and we apparently overlooked the point about living rooms multiple times.

The idea of a homebuilder making their margin by leaving out window area is unrealistic. "Window area according to DIN" or "lighting on the construction side" I have never seen in any construction performance specification even at discount builders.

In my opinion, some homebuilders advertise with standard floor plans that really only have the minimum amount of windows. Of course, that might be due to the price segment of the provider, likely so. For example, there are often kitchens that go around the corner but only have windows on one wall side, which are then also rather tiny. Our perception of the construction sector can of course be very selective here.

No one can take that off you in a few lines in a forum.
But approaching something via a minimum requirement of the state building code... are you an engineer at Audi or Siemens? ;)

Of course, the first thing we thought when we started planning our house was: first read the state building code and THEN see what we want / find beautiful / appropriate, you’re right about that!
The question here in the forum was about orientation, not about exact details (as described in the initial post), some of the answers have already helped to better classify things.

No normal builder reads a state building code at the beginning of their research – but only when something about their specific development plan strikes them as odd and they want to know how far the municipality’s zeal for regulation here goes beyond the standards of the state building code. I advise prospective builders in all federal states except Mallorca, and without a concrete reason, I have never read a state building code. It is a reference work, not a novel!

In my opinion, some home builders advertise with standard floor plans that really only have the minimum number of windows. Of course, this may be due to the price segment of the provider, that’s probably true. For example, there are often kitchens that run around a corner but only have windows on one wall side, which are also rather tiny. Of course, our perception of the construction sector here can be very selective.

I would rather call it looking through a warped lens. Not a single provider advertises floor plans that have only a minimum of windows (and I have already said, they have no interest in saving here at all). Catalogs advertise competitive models, this applies to all price segments. The "most cited" floor plans are those that are rated attractive and are often used as templates for individual plans. A provider who “skimped” on windows there would cut off their "marketing channel picture platforms."

For the moment, we will plan some currently planned windows a little larger and hold back money for any additional windows that we currently see as a nice variant so that we have enough leeway in the architect discussion.

You have – at least if you mean the draftsmen / technical sampling helpers of the providers – romantic ideas about "architect" discussions. Better show here what you have planned so far – then more concrete tips will come than you like ;-)

No normal builder reads a state building code

oh... is that so? I disagree.

I advise prospective builders in all federal states except Mallorca

but certainly not all!

This is a reference work, not a novel!

That's why you can still take a look at it, read it... even if it's only skimming.

In my opinion, some home builders advertise with standard floor plans that really only have the minimum number of windows.

They actually advertise houses with lots of extras. The valid scope of services description for the standard is usually much leaner. You probably mean the cheapest standard house of a series with a base price. The windows are perhaps the most visible position. However, in every trade the standard version is included; what you call comfort is available at an extra cost. Tip: there are also general contractors who do not charge for windows.