Single-family house with barrier-free granny flat on the ground floor

Correctly formulated, the task is to plan a two-generation house for six residents in two households, with the living unit for the grandparents being barrier-free.

we are currently facing the challenge of planning a single-family house for a family of four as well as a barrier-free granny flat on the ground floor for my parents.

This is the exact incorrect description of the task. It misguides in the essential point by treating the planning subset "granny flat requirement" separately and thereby tends to lead to precisely that extension, which is rather made worse by shifting the upper main part of the house.

It is difficult to recognize in the development plan. Are you even allowed to build 2 full floors + attic and 2 residential units? With your construction, parking spaces, and terraces, you clearly exceed the 0.4 floor area ratio. Is the exceedance permissible or is it excluded?

The design is not coherent to me. You are building a large ground floor so that the granny flat fits. On the upper floor you step back, leaving half as a flat roof "left over," and then you build the peak? Is it just me who finds that strange? Why not build the upper floor completely as a pitched roof with, for example, a knee wall of 1.40 m and omit the peak? Who wants to run around three floors senselessly when two are directly available and probably even cheaper?

And above all, when only 2 floors are possible... there is also something like the maximum ridge height.

Why not completely build over the upper floor as a gable roof with e.g. a knee wall of 1.40m

Probably because of the eaves height specification in the development plan of at least 5.50, maximum 7.00. There are contradictions in the development plan that have already been discussed "elsewhere." However, you are of course right, instead of a third floor simply simplify the cubature. And yes: in the development plan I read of no permission to increase the floor area ratio according to Par 9 Building Code and half for garages and access paths.

Well, with us the exceeding of the development plan was explicitly excluded. Therefore, I assume if nothing is stated, then it is permitted. Although it is somewhat comical to build 550m² with 2 residential units, 4 parking spaces, etc. but not want to "build". That simply doesn't work. Even an exceedance of the floor area ratio by 50% would have to be critically calculated. Parking spaces, access paths, terraces simply consume many m²... But build the granny flat in such a way that you can use it differently at some point, whatever that may be. Hoping that one of the children will move in... that rarely works out. The other point of criticism has already been mentioned. If real care is really supposed to be provided, a living space for "Magda will take care of it" is missing. True 24/7 care, no one can provide that unless they do it professionally for money.