How many windows should one have?

Hello everyone,

we just stumbled upon the amount of windows needed while planning our floor plan, depending on whether you:

[*]orient yourself on the state building code – according to the internet, supposedly 10-12% of the floor area should exist as window area
[*]orient yourself on newer recommendations like DIN EN 17037, which suggests 20-25% as a rule of thumb

Of course, we are aware that the lux calculation is much more complicated than the simple rule of thumb, but we want to calculate costs based on a reasonable number of windows so that we are not caught off guard later if we have to install windows worth €10,000-15,000 more.

Now we are asking ourselves how we could possibly even approach 20% in a 1.5-story building with a 20m² room. Even with two Velux windows with an effective window area of 1.16m² and a 160x140 double window (estimated then 100x110 effective window area after deduction of the frame, etc.), we only get 17.3%!
The same applies, for example, also downstairs in the closed kitchen, where with two windows measuring 120x100 (estimated then 90x70 effective window area after deduction of the frame, etc.), we would not even reach 10% – somehow I have the feeling that we still do not fully understand something here.

So now the question: how much window area (as a rule of thumb) should one really have, where is the sweet spot so that you wouldn't have to install 2 floor-to-ceiling windows and 10 Velux windows in every room. As I said, we are just trying to understand whether 20%-25% is really a realistic rule of thumb to try to achieve or if that might just be a lot of marketing, especially since Velux talks a lot about it on their websites. We just don’t want to build something like an old building in terms of lighting, and retrofitting windows afterwards sounds very exhausting.

Thanks to you all!

Light in the house is more than a percentage. Is there shading from somewhere? How are the windows oriented? What do you do in this room? Our large windows in the bedroom look good during the day, but I’m only in there briefly. At night, I don’t need large windows. It is also important to think about summer. Living spaces should have windows in two directions, so that you can shade the sunny side and still not sit in the dark.

Light in the house is more than a percentage [...] What do you do in this room? How are the windows oriented?

This is exactly about getting an idea as a layperson of what is appropriate where and how in which room, without having to forward every single variant directly to other people. These loops are simply tedious and slow when you are still considering how to arrange the windows at all. For this, we currently need a "simple" system that allows us to carry out an initial assessment ourselves. Unfortunately, I couldn’t find anything like that on the internet, hence the question here. As mentioned, it’s not about 100% accurate calculations but something where you can be sure that you are not planning complete rubbish that will later backfire in the lux calculation or when you live in it and sit in darkness, even if, for example, it was okay according to the building code.

That's why the layman is best off not planning anything at all, but going to the architect or taking a proven floor plan off the shelf. Or do you design your clothes and electrical appliances yourself as well? Amen.

Of course, I can also just take the standard floor plan from the house builder, then 10% of the [Landesbauordnung] requirements are met everywhere and the upper and ground floors only have 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, being able to discuss things as laypersons, and possibly bringing in special requests like more light. If they want to sell us only the minimum [Landesbauordnung] to save money but double that would definitely be better and we could save money elsewhere, then I want to be able to raise that.

For that, we currently need a "simple" system that allows us to make an initial assessment ourselves.

You are overcomplicating it. Even a layperson has a sense of whether a room will be bright or dark. If there is a need for discussion, it is best to sit down with the architect and talk about whether window X can still be enlarged or if a window can be added at location Y. The architect can answer that quite well by feel. An experienced architect also ensures that the windows of the different floors don't look haphazard but rather coherent.
Basically, you have a slight disadvantage with the 1.5-story house, of course. And also if you want a closed kitchen.