Honest opinion about the bungalow floor plan - approximately 175 sqm

Had calculated the total m2 with €1800/m2 (without garage and come to 315k) so €35k for a double garage is not realistic?

In Lower Saxony yes However, with €1800/m2 I would rather see the house type standard (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 properly 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 a buffer of 20% instead of 10%.
However, the €300,000 estimated by the architect is confusing, as architects include earthworks in their calculations.
I count these as additional construction costs.

The construction project will not be fully contracted out. Much will also be done with participation. The circle of friends consists of various trades. (Carpentry, electrical, interior finishing)

Please keep in mind that friends do not use your construction schedule as a private calendar.
Either you then have many construction breaks because the friendly electrician is only available the week after next, or you get nothing at all because private matters take precedence in the evenings.

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

The ridge direction is specified, and the two ridges of your house are transverse to the specification.

Good for you if I am wrong

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

Oh... I actually really like that, the "recess." The terrace can then be extended outward. Then you have both. The rooms gain very little sun through the windows, but light, yes. I happened to be virtually visiting the model home Okal Haus in the town of Stelle today. I often drive past it (terrace side), but unfortunately have never been inside.

1800 euros is too little. Better calculate with 2000 euros, that includes everything if you don't choose something completely extravagant.

That makes 35,000 euros more than planned. Better calculate 45,000.
Garage adds another 40,000.
Makes 85,000 euros.

The extra costs that come up during construction are reality.
I didn’t want to admit it either.

Here 2000 there 2000, after the construction period you end up with a significant amount.

A friend once said calculate 10% more, I laughed.

The result is he was right.

I see the point about the skylights, we are considering installing windows into the house walls there to get more light inside.

That sounds reasonable.

- You need to insulate the party WC more than currently drawn.

Double-shell overall 49 cm, where should that be insufficient?

Double-shell overall 49 cm, where should that be too little?

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