Single-story construction, room height 2/3 rule?

now we would like to build a "cube house" with a flat roof. Do I understand correctly that if the ground floor is 100m2 and the upper floor is 66.67m2, then the room height upstairs may be the same as downstairs?

As quoted by Backsteingothik, in this case it could even be up to 75 m2. But be sure to observe that if the development plan makes specifications regarding the maximum eaves height, these must of course also be complied with.

Ok, then I understand correctly that we could build it with the same ceiling height upstairs as downstairs? The development plan is from around 2020, according to the development plan a building height of 9.50m is possible. Then it would fit like that, downstairs about 100 m2 and upstairs about 66.67, the rest in the upper floor would then be laid out as a terrace.... Thank you

Theoretically, you have understood it correctly (assuming that you want to build in a two-thirds state), because the decisive factor for the full-floor question is the ratio of the above-230 areas between the floor in question and the reference floor.

In practice, I have the suspicion that you might have some naive assumptions "in your luggage," for example that a flat roof, if it is supposed to be watertight, would not be more expensive than a shed roof. You should also consider that a limitation of the eaves height / wall height (9.50 m reads suspiciously to me as ridge height) can practically mean that the top floor has to be set back on several or even all sides. This, in turn, causes substantial additional expenses for structural engineering and insulation.

thank you for the information. What do you think, does a flat roof 10x10m including concrete ceiling, insulation etc. cost 35000k?

regards