Cost framework and negotiation scope for prefabricated houses

Misunderstanding: €10,000 additional costs compared to the excavation when building on a slab foundation. We got this figure from the sales representative of Bien-Zenker - but of course we are happy to be proven wrong before the costs get out of hand...

I had understood you to realistically calculate a €10k difference between actual earthworks and the seller's estimate. Now I understand you to assume that a house with a basement would only be €10k more expensive than on a slab foundation? (unfortunately multiple question marks are erased here).

Is it perhaps to be understood that it is assumed that if one builds the house with a basement (overall significantly more expensive), then the additional costs for the excavation increase by 10k (compared to the costs for the excavation for the slab)?

Reads like this. However, the house builder assumes that the soil will remain on the property. Do you have use for 200, 300m³ of excavation?

So far we have no comparative values, but found the first offer from Bien-Zenker already quite "steep": For a turnkey Efficiency House 55 single-family home with a net living area of 165 sqm with two full floors and a hipped roof on a slab (without basement) and an air-to-water heat pump with underfloor heating, one would be looking at a transfer amounting to €484,000. The whole thing with minimal special equipment (floor-to-ceiling windows, electric shutters, partly parquet floors).

This results in an all-in price of just under €2,950/sqm (plus ancillary building costs, outdoor facilities and garage). With a basement, we would come out at €543,000 (just under €3,300/sqm).

Several remarks: 1. A hipped roof already causes an additional cost compared to a gable roof. Floor-to-ceiling windows and parquet flooring are also things that cause higher costs. Against this backdrop, it is nice that it stays below €3,000/sqm. 2. When comparing building companies, it is unfortunately indispensable to learn to read the building service specifications. On the one hand, unexpected additional costs are hidden in some building service descriptions that do not occur with other providers. On the other hand, the level of equipment sometimes differs drastically between different providers. Very different prices are definitely possible there. However, this can also be due to very different basic equipment in the end.

Be sure to "collect" building service specifications and compare them with each other. Whenever something does not appear or says "provided by builder," it ultimately remains your responsibility and causes additional costs.

When comparing construction providers, it is unfortunately essential to learn to read the construction service descriptions. [...]
Be sure to "collect" construction service descriptions and compare them with each other.

Basically, one should not just read the construction service description—especially with individual planning—but actively specify it. Even when building catalog houses, I advise my clients to have the construction carried out according to the construction service description of their independent architect.

On the other hand, the level of equipment also sometimes differs drastically between various providers.

In my opinion, this is the main benefit of the provider's construction service description: that it shows which equipment the provider considers "normal." This allows—if one is not practiced already in reading this from the marketing appearance—conclusions about which target group the provider’s desired customers are. Either you belong to that group yourself, or you look for another provider. With Bien-Zenker, Schwabenhaus, and Schwörerhaus, the OP has named examples of competitors that are reasonably comparable with each other. To these three wood builders, I would add two or three masonry builders as a further option in a choice decision.

Misunderstanding: €10,000 additional costs compared to excavation for slab-on-grade construction.
We got this value from the sales representative of Bien-Zenker – but of course, we are happy to be corrected before the costs get out of hand...

That means that with a slab-on-grade you only excavate one meter, your 2.50 costs €10,000 more.
And this is only about the excavation: in the construction service description it is very likely, as others also handle it, that removal and disposal of the existing soil is not regulated. That generally costs extra again. And keep in mind: excavated soil has at least three times the volume of undisturbed soil.