Planning guest WC in new construction - How big should it be? (DIN?)

It will be enough for a desk and a guest bed. Currently, there is a 35-degree roof pitch and a room height of 2.30m in the middle.

Is the ridge beam and floor construction already taken into account?

Dormers are certainly nice, I stood in the attic with the freshly installed roof truss and thought I would like to have one too. But every dormer costs a lot of money and with each one you get closer and closer to the cellar. I know you have decided that a cellar is not an option. But on the small plot, it would really be helpful, and if you are building without one for financial reasons, you should not spend the money again on the attic.

I believe that has already been taken into account. But I will check again. We will not build a structure costing €80,000 on it now. The basement is roughly in that size range, rather more. We will wait for the offers. By the end of the week, I expect feedback from the last two construction companies still in the running.

A dormer doesn't cost the earth either and is definitely significantly cheaper than a basement given the construction prices.

The dormer also delivers less. And then you have to see it in the overall package. We should not talk about placing exactly the same house once on a basement and once on a slab. Using the attic means here necessarily a upper floor without sloping ceilings. In other words, with a basement you can not only save the dormer but also a few rows of bricks, thus also plaster, etc., and that adds up (I currently have about €1500 per 10cm knee wall increase in mind). In addition, you save building height and setback area. That might cost about €40k more in the overall calculation, but it has more space and more flexibility.

A basement also does not provide any light. And for working there, a room is only attractive through large windows. And then you are very quickly at an expensive living basement for 80k.

Using the attic here means necessarily a full upper floor without sloping ceilings.

The original poster, Till, is building two stories, just like you. On top of that comes a roof without KS… roughly like yours.
This is basically about the "expansion reserve" in the attic, which unfortunately is not as pronounced for the house size as yours.
The space in the sitting attic only becomes a room through planned dormers.
Sometimes it’s really sad how tight and small you are allowed to build.