Massive or wood / Drilling or trench collector

Thank you for your reply.

I think then the small toilet upstairs will be removed and where the office is now upstairs, the children's bathroom will be put in. This office is not really necessary. Only the one downstairs.

Do you spend most of your time in the bathroom, or why do you need a whole 3 bathrooms for 4 people?

Would give the bedroom a shadier spot and more light to the children. Measure exactly how big your bed is. Looks a bit tight in the bay window. What is the hobby room for? I find the access through the bedroom unfortunate. On the ground floor, remove the shower so that the 80s slant disappears. 3 full bathrooms for 4 people is quite generous. I find the wardrobe under the stairs not fitting the house. It seems like a makeshift solution. Impact sound depends on the floor structure. Floating screed, fill layer, etc. I will install solid wood in the room. The cable ducts, etc. are neatly milled out and you can do everything yourself.

Just asking the experts here: Obviously, the walls are not aligned. Could you build it that way, or does it make things very complicated?

Well, it is especially expensive...

that's not a problem then the hallway upstairs will be narrower and the small toilet will be removed. And the utility room with bathroom will shift slightly to the right and become a bit smaller. The staircase is angled, so the part towards the front door will be a bit longer.

Wardrobe should be under the stairs

I have now done a bit of calculation regarding the stair size. If we conservatively assume a floor height of 2.85m and let the stairs run 1.5m around the corner at the bottom (more is not possible because of the front door), then the other side must be 3.8m long. The basis are 16 steps with a rise of 17.8cm, depth 26.3cm, tread length 61.9cm. With 15 steps you could still squeeze a bit, but then the stairs will get really steep! 3.8m compared to 2.8m is a big difference.

The wardrobe under the stairs will be very small, because where you actually have room there is already the door to the office.