Single-family house approx. 150 sqm floor plan - How to plan the staircase?

There will be an additional 1.5-2 cm of plaster on each wall.

Another question: does the development plan mandate single-story buildings?

A full floor... but I actually assume that with a 1 m knee wall despite the captain's gable we do not reach a second full floor, or am I mistaken?

Unfortunately, I can't see where you are building in Tapatalk, but I find it hard to believe that your house is not considered two-story.

Schleswig-Holstein

Definition from the state building code states: Full storeys are above-ground storeys if they have a height of at least 2.30 m over at least three quarters of their floor area. A top storey set back from at least one exterior wall of the building or a storey with at least one pitched roof surface is a full storey if it has a height of at least 2.30 m over at least three quarters of the floor area of the storey below; the height of the storeys is measured from the top edge of the floor to the top edge of the floor of the ceiling above, in storeys with roof surfaces up to the top edge of the roof covering.

The sloping ceilings are not shown in the first floor plan.