What is the best process for making windows in this type of panel for house?
Thickness of the panel is 28mm
That would depend on how you have modeled the panel.
Attaching an example model would help.
If you are interested in a plugin… Look at Profile Builder 3, it has an impressive cut opening tool
Profil Builder is Great !
But in your case, i think you should look at this.
1001bit Plugin (at 34:15)
Having looked at your model it appears to me it would help if you took a bit of time to understand Groups and Component better and how to Intersect faces in context.
Here is a short version of how I would construct your model, I’ve only shown one wall to fit it in the limit of a gif. Note the floor is a separate group from the walls and by using a component I reduce the amount of editing needed to work with your slats while still making it possible to have different windows in different walls.
There are many extensions that can help you speed up your work, but it is important to understand the underlying geometry.