Honest opinion about the bungalow floor plan - approximately 175 sqm

In Lower Saxony already. However, at €1800/sqm I would rather see the standard house type (rectangular and gable roof).
Shed roof is more expensive, flat roof is more expensive, then you have two connections (flat roof/wall) that must be correctly executed to keep the house dry.
More work for the roofer, etc.
Then an insulated garage... oh dear... then you are close to 400,000.

With architects, always take 20% instead of 10% as a buffer.
However, the €300,000 estimated by the architect is confusing, as architects calculate including earthworks.
I count these as incidental construction costs.

Please keep in mind that friends should not use your construction schedule as their private calendar. Either you have long construction breaks because the electrician friend only has time the week after next, or you come up empty-handed because private matters take priority after work.

Regarding the house: I see it like the others, the house gets very little from its positive west side, the hallway is very, very long, the children's rooms are not exactly fairly divided. The shower and toilet should be swapped so that you have natural light and a view while sitting. The T needs to be slightly offset. The wardrobe is no longer sufficient for 4 people, for 2 it’s just enough seasonally. And where do the other jackets, bags etc. go? The hallway is downright wasteful. The utility room is nicely large but has to serve many purposes. For the next planning you can include all criteria, because you won't get around a re-planning if the development plan is still valid:

The ridge direction is specified, and the two ridges in your house are perpendicular to the requirement.

Good for you if I’m wrong

I agree with Yvonne. Also: The atrium-like recess to the garden will ultimately only become a dark, cramped corner. I would rather sit on a wide sunny terrace.

The garage is certainly unheated and I don’t see a 49cm wall there either.

I always had the impression that a shed roof and a flat roof are rather cheaper than a gable roof.

Children also come one after another, which is why we decided on a smaller and a larger one.

Since we live directly next to the construction site in a house we bought, we don’t really have any stress there. It can also take a year sometimes.