Planned new single-family house - floor plan available

Only then would I continue planning

It says that the roof of the house must be a gable or a half-hipped roof; and if one wants to have the garage or the carport including the garbage bin and lawnmower outbuilding with a flat roof instead, it has to be greened. Apparently, as a layman (but why the contractor too?), you have misinterpreted this, thinking that with green roofing the main roof can also be flat – but you won't get away with that.

I don't necessarily see it as clearly as you do. Unlike the OP, you highlighted the other part of the sentence in bold; if you read it as in the OP’s questionnaire, the exception is not limited to an outbuilding:
"As roof types, gable and half-hipped roofs are permitted for the main roof surfaces – garages and auxiliary buildings and greened roofs are exempt from this."

According to your interpretation, it should rather say ...garages and auxiliary buildings with green roofs... or something similar.

If the authority's answer is positive, then:

I don’t think the ground floor is bad, relatively generous entrance area, good orientation of the rooms, staircase not in the dirty area... fits.
What I would change: remove the pantry and enlarge the kitchen, enlarge the guest WC to a real children’s bathroom. If the two children’s rooms are to be put to use, they are simply too small!

I can well understand the wish for a separate "parents" area; we also tried to solve that at least in principle, and every evening when I open the door to go to bed I am happy about the separated zone including the ensuite bathroom... it works.
But: please arrange it differently! The corridor will never be a dressing room, it will always remain a corridor. Given these space conditions, there should be a better solution. Do you need a 12 sqm bathroom for two people? Probably not. You can well take something from it and add it to the dressing room, but overall please do it differently...

Last but unfortunately not least: the budget will not be enough. At least with this equipment and building form I don’t see 450k all in. Do you have a buffer or can you do some work yourselves? Then it might (wasn’t it Lower Saxony, where it is somewhat cheaper in the countryside?!) be possible.

Only then would I continue planning

Everything else makes little, or rather no sense at all.
It's just interesting that the construction company and an architect, with whom we also had a conversation beforehand, understood it that way. I am not to blame - I am an amateur...

I don't necessarily see it as clearly as you do. Unlike the OP, you have highlighted the other part of the sentence in bold, but if you read it as in the OP's questionnaire, the exception is not limited to an ancillary building: "For the main roof surfaces, saddle roofs and half-hipped roofs are permitted – garages and ancillary facilities and green roofs are exempt from this." According to your interpretation, it should rather say ...garages and ancillary facilities with green roofs... or something similar.

You are right that your reading cannot be dismissed either. Both are possible, and the municipality itself knows best what intent drove them when drafting the specification. The question is therefore: what does the builder want? If the builder wants a flat roof, it should be a straightforward matter legally to read the permission here – provided one is willing to green the roof. If the builder wants to build in conformity with the spirit of the development plan issuer, only a written preliminary clarification offers certainty. I simply based myself on my experience of what development plans usually desire in such cases, and that would be a landscape-typical building with as little "Bauhaus style" as possible; among other things, I read saddle or half-hipped roofs as a hint (also that hip/hipped or tent roofs like on the common corner villa are not desired – and presumably, in rural Lower Saxony, not even the grand Hamburg-style coffee grinder). Increasingly, development plan issuers pay attention not to drive away local plants like by ostentatious nouveau riche insertion certificates of poverty. It is clear in any case what the term "main roof surfaces" means: namely the design-dominant part of the roof surfaces of the main building. This means that the roof form specification excludes secondary roofs (of bay windows, dormers / cross-gables) as well as roof surfaces, especially of usually boundary-privileged ancillary buildings such as carports. Regardless of the municipality’s answer, I would like to emphasize two already voiced points again: 2. every projection and recess and every non-aligned wall costs; 1. a fragmented cubature – starting with the extremely unbalanced surface distribution on ground floor and upper floor – runs counter to an energy-efficient building envelope.

Is the income/equity sufficient for all costs?

For the main roof areas, gable and half-hipped roofs are permitted as roof types - garages and ancillary facilities and green roofs are exempt from this.

Do we have or is there an exact wording of the development plan?
I agree with .

Every second development plan has this statement regarding the green flat garage roof...

And this statement:

that use as a residential garden is not permitted within the 3 m boundary offset area.

...Implies the boundary development, which does not apply to residential or main houses, but only to garages, which are often used as "residential garden/roof terrace".