Single-story construction - is it possible to plan a dormer on the upper floor?

This is not wrong,

For the floor space index it is wrong, for the rest at least misleading.

Where the states are, as far as I know, in agreement, is on 2.3 m Geschoss height

There is no agreement. While, for example, Bavaria and Brandenburg have removed the full storey concept from their state building codes (it only appears in transitional provisions), NRW has switched from storey height to clear room height (currently not law due to a moratorium).

Where can I read this? In the development plan?

If it is not about whether you should buy the property or not, but about the best possible approach to your dream house, you should discuss the details with your planner. Many regulations may still be hidden in the development plan that affect the design of the roof.