Is a second shower bath sensible?

I briefly considered a shower in the guest WC as well, but I'm not sure if that would be possible. The guest WC is 2 sqm, behind it is the staircase and in front is the hallway, which is not very large either. Unfortunately, we have not yet had a detailed conversation with the technical internal service, but this is supposed to happen soon.

My parents renovated their second bathroom after 25 years - after almost all the children were out

I didn’t like having just one bathroom, but I also have four sisters. A lot had to be coordinated in the mornings.

Therefore, we have three shower bathrooms plus a guest WC, and I already enjoy having a toilet on every floor

So, I would not consider converting the guest WC in the [REH] with a shower on 2sqm, which cannot be significantly expanded. I would probably make the preparations and plan the attic accordingly. The possibly future bathroom can then be used as a storage room for the time being.

I can still remember when we were kids in a 4-person household with 1 bathroom. Everyone had to leave the house at the same time... back then, you had to get up early, etc. No, not anymore, those few euros for the 2nd bathroom were worth it to me.

So I grew up with one bathroom and it worked – I don't think it even bothered me much. Even the 3 years in a rented apartment weren't as inhumane as it sounds from some people here. However, it is more comfortable with 2 showers.

Had to switch, I guess I'm not that fast at replying with my phone

for points 1-3 without tiles it would be 7.5k. The tiles at standard for just under 4sqm are 1.8k.
So a good amount for that. We, or rather I, think that if it currently works only with a small bathroom, then it should also be enough in the new building with a much larger bathroom and a guest WC.
We mostly stay downstairs or on the upper floor, the attic would be virtually unused for the first years.