Construction costs for a single-family house with a basement in NRW

Insulation is more expensive in the basement because it is the same thickness but made of XPS.
Facing brick is omitted, yes, but you have an additional cost of 20-30K€ for earthworks.

The comparison is fine. Additional cost for facing brick is 10K€, ventilation is more expensive for you because of more area (3 floors) and you wanted an air conditioning system as well.

But this is not about haggling with you. I am not your potential contracting partner. Then just deduct 10-20K€ from my example if you consider it not comparable. But that doesn’t change the fact that you are 100,000€ off.
Only serious project-related calculations and concrete offers can prove this concretely. Plus the knowledge of whether they are really complete.
You have your plot of land. Just go to a general contractor or architect and get an assessment there.

Insulation in the basement is more expensive because it is done with the same thickness but using XPS.
Brick façade is omitted, yes, but you have 20-30k€ additional costs for earthworks.

The comparison is okay. Extra cost for brick façade is 10k€, ventilation is more expensive for you because of more area (3 floors) and you also wanted air conditioning.

But this is not about haggling with you. I am not your potential contract partner. Then just deduct 10-20k€ from my example if you consider it not comparable. But that does not change the fact that you are off by 100,000€.
Only serious project-specific calculations and concrete offers can prove this. Along with the knowledge whether they are really complete.
You have your plot of land, right? Just go to a general contractor or architect and get an estimate there.

I will do that as well. Appointments with architects have already been made. But I need to know roughly what the prices are.

I guess I will keep the plot for speculation and buy one of the houses developed here from the property developer. They start at 350k without a basement but including the land. For 170 sqm of living space.
That still makes me cheaper.

Remember that you have to pay the ancillary purchase costs on the entire purchase price. At 350,000€, it rarely stays that way thanks to added expenses. Hold onto the land in reserve ... respect for this financial background. Utilizing or at least mortgaging the land would probably be the reality for most home builders. 170sqm above ground is quite nice. But it is 50sqm less than your last wish. It helps you considerably with the budget, though - it becomes more realistic.

We built a house in the Viersen area 2 years ago and spent about €450,000 excluding the land, with the following features (maybe this helps as a reference):

1.5-story single-family house with gable roof, brick-clad
KFW55
160 sqm living space
80 sqm basement with waterproof tank
Entire house tiled
3 bathrooms (2x walk-in showers)
Double garage
Ventilation system
KNX
Geothermal
Outdoor facilities (without planting)

Regards
Urmel

Two years ago we built a house in the Viersen area and ended up at about €450,000 excluding the land, with the following features (maybe this helps as a reference):

1.5-storey single-family house with a gable roof, brick-clad
KFW55
160 sqm living space
80 sqm basement with waterproofed concrete
entire house tiled
3 bathrooms (2x walk-in showers)
double garage
ventilation system
KNX
geothermal energy
outdoor facilities (excluding planting)

Regards
Urmel

Hello. Thanks for the numbers.

What were the prices/surcharges for KfW 55, KNX, geothermal energy?

Best regards

You can probably calculate geothermal energy at about 10k, the better insulation costs 5k, and for KNX you can budget a generous 20k.

So you can deduct 35 from the 450.

We wanted a basement and, with a floor area of about 10x10m and as a utility basement inside the thermal envelope—because that’s what it basically comes down to—we should have expected additional costs of 80k. Also as a waterproof shell, not a habitable basement.

The prices mentioned here from all sorts of different sources are therefore not entirely unrealistic.

Please also consider that statistics very often lack things or generalize. Such things as social housing can also be included.

Please consider that anything that is not absolutely basic standard moves you from simple to medium or high standard.

Example:
Simple: White double-glazed plastic windows
Medium: Colored double-glazed plastic windows with manual shutters
High: Through-colored triple-glazed floor-to-ceiling windows with electric shutters

Simple: Kaldewei bathtub
Medium: Kaldewei tub + shower with plastic partition
High: Kaldewei corner tub (long) + floor-to-ceiling shower in a niche with glass door

Simple: 50 sockets, 8 switches, 2 TVs
Medium: 80 sockets, 10 switches, 4 TVs
High: 120 sockets, 14 switches, 7 TVs
Not to mention the switch program... there are huge price differences between SI reflex, BJ AS500, and many many other designs.