Why building a house almost always costs more than calculated

You have demands - which you of course fulfill yourself, right?

Unfortunately, I am not building new, but have to renovate an old building. And yes, I spend money beforehand so as not to be left empty-handed afterwards and also put in the effort. Basically, a lot is questioned, whether it is sensible, necessary, plausible, alternatives, etc.

- awg - the sugarcoating and ignoring of defects/problems until there is no other way <- has not yet been addressed....

I would simply say that projects cannot be planned strictly according to waterfall on day 0. With a GÜ, one buys insurance against this (if the construction service description is accurate).

Renovation is a completely different matter - I would never put myself through it, it involves even more risks.

Renovation is a completely different matter - I would never put myself through it, it carries even more risks.

Unfortunately, I have no other option than to buy an existing building and renovate it for my purposes. It is definitely expensive.

Due to extremely heavy rainfall, additional work became necessary for us. Here is an example of additional earthworks on the cellar beyond the standard service:

Disposal of 600m³ excavation: €8000
150m³ fill material: €1050
Pump shaft for open water removal including pump rental: €1500
Light shaft emergency drainage: €1500
Construction site access and crane area: €1200
Soil survey with foundation recommendation due to questionable load-bearing capacity after rain: €1200
Geotextile and 15 cm additional gravel layer under the floor slab: approx. €1800

For additional costs due to earthworks etc., we calculated a buffer of approx. €20,000. If the construction company hadn’t passed on the wholesale prices to us, it would probably have been a close call.

I think this is a point that many underestimate. Our landfill/fill material was relatively inexpensive. That can also be significantly more expensive.

Install geotextile and 15 cm additional gravel layer under the floor slab: approx. 1800€

This point puzzles me a bit:
My wife is a landscape gardener and has also purchased geotextile for our property.
The same geotextile is also used in gardening and landscaping for slope stabilization. It has almost nothing to do with the normal weed control fabric. We bought 180 g/m². The fabric costs less than 100 euros per roll of 25 m x 4 m. There is also this fabric with 300 g/m² – the price is 170 euros. I just visited the company’s website again. The 300 g/m² is the strongest the company offers. It is sold not only in the agricultural sector but also in road construction and building.

I can’t really assess which fabric your construction company used and how much the gravel and labor costs were – but I am quite surprised by the price.