Single-family house 172 sqm with garage and sauna

Nds is cheap building land:

180sqm x 1900= 342000 Energy Saving Ordinance 2016 (Viebrockhaus itself is more expensive, but I am not sure... you can easily be mistaken there )
Outbuildings 25000 +
Sauna 5000
Fireplace 8-10000 AI
Floor coverings/painting in EL 20000

= approx. 400000€

Plus outdoor facilities +- 20000
Plus incidental building costs of 25000 (flat land with good soil)

I find the dressing room poorly designed. How wide is the room? 2.24? So in the end, only 1m narrow passage remains, since at least 2x 60cm are taken up by the wardrobes. This means you end up walking back and forth 5 times between the bedroom and the dressing room to pick out and put on the right clothes... This can be done better. The budget is not sufficient, but that has already been mentioned. Is that actually your budget just for the house and you calculate additional construction costs separately, or are those already included?

No, you are interpreting that exactly right!

Enlighten us: what is the catch of a transit guest WC?

Based on Viebrockhaus Maxime 800

I recommend letting go of simply copying a template. It does not succeed, as can be seen.

Which wishes were implemented by the architect? – so far everything exactly as we drew it....

More likely from the rubber stamp of a general contractor. Any independent architect would act in an advisory capacity here (i.e. also: dissuasive) if the client planned nonsense, instead of blindly copying it 1:1. At least in the knee wall behind the bedroom, this should be evident even to laypeople, unless there is an essential lack of spatial imagination.

Who is responsible for the planning: self-drawn, architect digitized. BUT: so far only the dimensions are set; interior walls, windows, and furniture are partly not yet in the right place!
Where do the dimensions even come from if furniture has not yet contributed to checking the proportions?

Style - bungalow area, old house still standing, will be completely demolished. [...] - eaves height of the single-storey main buildings must not exceed 2.75 - 3.00 m (measured from the top edge of the plinth to the bottom edge of the gutter) [...] further requirements - originally a designated bungalow area [...] The sauna is deliberately located in the 2.20 m high garage because an immediate exit to the garden is important to us.

Does the bungalow have a basement? - I would think thrice about deep demolition. With this low eaves height, it will be tight for the garage to slip under the roof overhang with only one step difference. From the demolition, I conclude that no infill development is planned here, but rather the character of the area is to be preserved, so there are rather poor chances for exceptions that would oppose this.

Lower Saxony is cheap building land: 180sqm x 1900= 342000 Energy Saving Ordinance 2016 (Viebrockhaus itself is more expensive, but I'm not sure... one can also be quite wrong about that ) Outbuildings 25000 + Sauna 5000 Fireplace 8-10000 AI Floor coverings/painting in EL 20000 = approx. 400000€ Plus exterior facilities +- 20000 Plus incidental construction costs of 25000 (flat land with good soil)

Hm....now you have painted a little "P" in front of my eyes

Budget is not sufficient, but that has already been mentioned. Is that really your budget just for the house and you calculate ancillary building costs separately, or are they already included there?

That should actually be our budget for the house and we would calculate the ancillary building costs on top of that, so plan them additionally.