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

Here is an example of a construction project in Hamburg from 2 years ago. The house is now standing. They cleverly maximized the living space on a plot with dimensions similar to ours. In particular, the two dormers turned a narrow attic into a very usable space.

Visually not a highlight (in your picture),

I couldn’t have put it better ;-) Now Günni has to make a Doornkaat for me without a back from that awful example picture *LOL*

A dormer doesn’t cost the earth and with construction prices it’s certainly much cheaper than a basement.

Not everything that limps is a comparison ;-) A little dormer rarely comes alone. In practice, reinforcements are usually added because the thing spans several rafter fields, and that’s just the beginning of several verses of extra costs.

I would much rather have a dormer than a basement. Regardless of the costs, the space created is much more attractive than a basement room. If you have doubts about dormers, then take a look around the Netherlands. They have built so small in the cities that really every house has a proper dormer. It looks okay to good and makes something really usable out of a small house. I would have a dormer built if it is financially and technically feasible. Regardless of how much a basement would cost.

A basement also doesn't provide any light. And for working, a room there is only attractive with large windows. And then you're quickly at an expensive living basement for 80k.

You overlook the technical room that moves away from the ground floor. Completely without a living basement. A lower building height might also allow for half a meter or even a meter more house. I basically have the same concept as the OP, albeit with a 30-degree roof pitch. Hence the question whether the purlin and floor construction are included in the 2.30 m height. If an additional 30-40 cm for the purlin and 10-15 cm for the floor construction are added, it becomes significantly less generous than the plan with raw construction dimensions suggests. I once considered extending the actual staircase to the attic instead of the pull-down ladder. For that to fit in terms of height, a dormer would have been necessary. The general contractor mentioned around 15k as an estimate for that. If you make it a bit bigger and maybe two of them...

You are overlooking the technical room that is moving away from the ground floor. Completely without a basement.

Or, if the attic can still be "relatively spacious" here: one is considering relocating the technical room there, see:

One could also only use the attic for storage and provide a connection room – so non-habitable rooms, and consequently without elaborate escape routes. We discussed this with @Golfi90 in

A steeper roof would also improve the possibility of using the roof. Max 2.3m height is too low.

Would definitely do it, but the building regulations are not a wish concert. The current dimensions are not a coincidence but correspond to the maximum allowed dimensions. It will be enough for a desk and a guest bed.