Assessment of construction costs for a single-family house in NRW (current situation)

Exterior walls + insulation + interior walls are calculated with a thickness of 0.4 in our case.
-This results in a reduction of the interior area by about 17-18 sqm ...

So it is approximately 143 sqm, including hallway / stairwell

Exterior walls + insulation + interior walls are calculated with a thickness of 0.4 in our case.
-This results in a deduction of the interior area of about 17-18 sqm ...

So it is approximately 143 sqm, including hallway / stairwell

No matter how often you try to mentally dismiss our experiences, it changes nothing.

Exterior walls + insulation + interior walls are calculated with a thickness of 0.4 for us.
-This results in a reduction of the interior space by about 17-18 sqm...

So it's about ~143 sqm, including hallway/staircase

you want to build 9x12. that's 108 sqm of floor area and minus the walls you have 90 left. Plus the upper floor, where you almost have a full story because of the dormers, and the basement room. How do you get only 143?

by the way: dormers are really expensive. We also have 2 of them.

I also want to build new. I have declared some rooms as hallway and thus only come to 45sqm living space. Do I then also have to pay for the "hallway"? Warning: Anyone who finds irony may keep it...

Summary of contributions again:
House: 3000*150sqm = €450,000
KNX: €50,000
Photovoltaics: €15,000 (possibly without storage)
Outdoor facilities: €45,000
= €560k

Additional construction costs: €50,000
= €610k

Adjustment points:
Basement: €100,000 (no basement needed if it’s just for an office, maybe the upper floor would be bigger, for that you’d need floor plans…)
Garage: €10,000 (maybe it will just be a carport then)
Climate: €10,000 (prepare core drilling, devices possibly later)
= €730k

Not included
- Architect fees (incl. site management): €55,000 (if he really bills according to HOAI… you already have one, what does he say about his fee and the colleague structural engineer’s?)
- Kitchen, lamps, furniture, etc.

Why does it have to be €600k at all costs? You have €300k equity and a plot, so the income shouldn’t be below €4k? Then a higher loan might be possible!?

I am asking if there is a slope. If the question has already been answered, please provide the post. I do not want to read all 40 posts.