Basic questions about floor plan design (thickness of walls, etc.)

On the screen it actually works worse, but I only switch there once the error-attempt-crumple round at the scribble note level is dealt with. And at the scribble note level that works great: 9.5 x 12 m external dimensions become 8.3 x 10.8 m budget for the room width sums, i.e. in "1:100" I would have to allocate 16.6 x 21.6 calculation boxes there. Roughly I calculate 2x 36.5 plus 1x 24 plus 2x 11.5 equals 120, which comes close to about 150 sqm base plate as a "rule of thumb".

That on "the other side of the corridor" there is a roughly different number of intermediate walls along the route is rather atypical - and that usually evens out well for me by never "sewing measurements tightly". That the transcription from preliminary draft to design had gotten stuck on that never happened to me.

You can use measurements from your apartment.

If the wardrobe niche is optimal, use it.
If the bedroom closet could be a bit bigger, then plan for more closet space.
That is also more important than wall thicknesses. You need to know
3 meters of closet space is too little.
The 50 cm distance between bed and wall is too little.
Use the optimal distance between sofa and TV, etc.

This way you can create a room program with your individual requirements.

So don’t worry about the "single units" of centimeters and feel free to shamelessly think in the more layman-friendly ten- or five-centimeter steps of the decimal system (and when it comes to tolerances, always use bold five cm increments, even where in reality it might only be two). The professional planner can translate that into an eighth-of-a-meter grid.

Yes, we have now started to plan using Excel (yes, it works with that too...). As a basis, we assumed squares of 5cm x 5cm.
In this first planning phase, what matters to us is whether we should plan a 10m x 10m or a 20m x 20m house based on our space and room requirements ;-)...

Graph paper, ruler, pencil, eraser
Is better than Excel

Do you have a plot of land?
First, roughly outline your budget

Create a room program
Look at model houses. What do you like, what not

Take the floor plan of the house you liked best. Then see if you can fit your wishes in there

This gives you a rough direction.

A floor plan without land is useless

In our initial planning, we are concerned with whether we should design a 10mx10m or a 20mx20m house based on our space and room requirements ;-)...

20x20m is definitely more suitable if it should be square – because you actually need more space: "square" just like "symmetrical" only works in connection with the willingness to add a certain "area ballast" to balance the scale.

As far as "10x10m" is meant in the sense of "100 sqm": this slab base area would translate into about 160 to 165 sqm (two-story) or about 125 to 130 sqm ("one-and-a-half-story") of living / usable space.