ᐅ How many windows should a house have?

Hello everyone,

We are currently planning our floor layout and have come across differing recommendations regarding the amount of window area needed, depending on whether you:

• follow the national building code – according to information online, this would suggest having about 10-12% of the floor area as window area
• follow newer guidelines such as DIN EN 17037, which recommend 20-25% as a rough rule of thumb

Of course, we understand that calculating illuminance (lux) is much more complex than just a simple rule of thumb, but for budgeting purposes, we want to estimate a reasonable number of windows upfront to avoid being caught off guard later when we might need to add windows costing an additional $10,000–15,000.

So, we are wondering how a roughly 20m² (215 sq ft) room in a 1.5-story house could possibly meet a 20% window-to-floor area ratio. Even with two skylights (Velux) providing an effective window area of 1.16m² (12.5 sq ft) and a 160 x 140 cm (63 x 55 inches) double window (estimated at about 100 x 110 cm (39 x 43 inches) effective window area after accounting for frames, etc.), we only reach 17.3%! The same applies to the closed kitchen downstairs, where two windows each measuring 120 x 100 cm (47 x 39 inches), estimated at about 90 x 70 cm (35 x 28 inches) effective window area after frame deductions, wouldn’t even reach 10% – somehow it feels like we might be missing something here.

So the question is: how much window area (as a rule of thumb) should you really aim for? Where is the sweet spot so that you don’t have to install two floor-to-ceiling windows and 10 skylights in every room? As mentioned, we just want to understand whether 20-25% is a realistic guideline to aim for, or if it’s mostly marketing hype — especially since Velux tends to promote these figures heavily on their websites. We want to avoid building a dark, poorly lit home, and retrofitting windows later sounds like a real hassle.

Thanks a lot!

Light in a house is more than just a percentage. Is there any shading from somewhere? How are the windows oriented? What is the purpose of the room?
Our large bedroom windows look good during the day, but I only spend a short time there. At night, I don’t need big windows.
It is also important to consider summer. Living spaces should have windows on two sides so you can shade the sunny side while still having natural light.

WilderSueden schrieb:

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

This is exactly about getting an idea as a non-expert of what is appropriate in which room without having to forward every single option to other people right away. These loops are simply tedious and slow when you are still considering how to arrange the windows in the first place. For that, we currently need a “simple” system that allows us to make an initial assessment ourselves. Unfortunately, I couldn’t find anything like that online, which is why I’m asking here. As mentioned, this is not about 100% accurate calculations but about having enough confidence not to design something completely impractical that will later cause problems in the daylighting calculations or when living in the house and sitting in darkness, even if it technically fulfills, for example, the requirements of the building code/planning permission.

That’s why it’s best for amateurs not to plan anything themselves but to go to an architect or use a proven standard floor plan. Or do you design your clothes and electrical appliances yourself?
Amen.

Of course, I could just go with the builder’s standard floor plan, which only meets the building code minimums by about 10%, and has only windows facing one direction per room on the upper and lower floors 🙂 But that’s not how I want to live. It’s not about designing everything ourselves, but about being able to communicate effectively, discuss things knowledgeably as non-experts, and suggest special requests like more natural light. If the builder wants to sell us only the minimum code requirements to save money, even though doubling that would be clearly better and we could save money elsewhere, then I want to be able to bring that up.

roookeee schrieb:

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

You are overcomplicating it. Even a non-expert can get a sense of whether a room will be bright or dark. If there is a need for discussion, it’s best to sit down with the architect and talk about whether window X can be enlarged or if an additional window can be added in spot Y. The architect can usually answer that well based on experience. An experienced architect will also make sure that the windows on different floors don’t look randomly placed but appear harmonious.
Basically, in a one-and-a-half-story house you do have a slight disadvantage. And also if you want a closed kitchen.