Cost reasons. 1 sqm of basement costs us about €1,250, 1 sqm upstairs about 3 times more. We do not plan to use the basement as a living space but for sports, music, and crafts.
That’s nonsense – more accurate is:
As a pure utility basement, that’s about right.
The standard of the finish determines the price; there is no discount for the underground location. You can also hang a surface-mounted wired pit lamp on the unfinished ceiling in above-ground rooms (watch out for the pun).
Sorry, I expressed myself unclearly. I did not want to increase the number, but rather find the "right" 3 timber and masonry builders and, if possible, not arbitrarily get any 3 offers. As a layperson, I can only objectively narrow the selection by "builds in my region," "obviously above or below my budget," and "according to the website, has a solid wall structure."
But you have already contacted the three timber builders, with a draft plan (probably because my housebuilding roadmap is still not required reading at German building schools). That was unwise, and at this stage, at least I no longer take on such mandates, at least not without significant pain money surcharge. Therefore, I now see two obvious paths for you: A. You do the same again, but with a fourth timber builder and two masons. Then you can hopefully judge yourself which of the timber builders to eliminate, and have two masons as "test candidates" to see if the offers would still be similar if the construction method were changed. Or B. You go to one of my timber-focused colleagues (Beuler aka prefab house expert or Freyermuth aka prefab house Guido; in Lower Saxony I would consider it nonsense to send you to colleague Zink in Bavaria) and have them evaluate the offers you have.
The architect advised me to rather keep my distance from the very big players, since building according to one’s own plan/layout apparently doesn’t work well there? That’s why I concentrated more on medium-sized companies.
The advice is basically correct, even if the reasoning is absolute pub talk quality (common nonsense).
With smaller providers (<100 houses per year) I would be worried about the insolvency risk (although with the bigger providers you are not entirely safe either).
The economist Wolfgang Grupp Sr. says (paraphrased) that you recognize failures by the lack of owner liability. The best general contractors are owner-managed and build roughly as many houses with roughly as many permanent employees as weeks in a year – this applies regardless of construction method. I view the small carpentry firms with caution because in my opinion they are operationally (not to be confused with product quality!) at the level of their nationally known competitors half a century ago. Anyone who wants their wall structure "by the pharmacist for pharmaceutical retail" is (only) right with them; conversely, they then have to forget the big names. Your suitable general contractor as timber builder might have only about 30 own staff, as mason (if also active as general contractor) easily 250.
But regarding the use, I don’t see the aforementioned price. In the sports room, you should be able to ventilate properly, possibly automatically. The often reduced ceiling heights in the basement don’t always suit sports use (e.g., treadmill). Heating is also an issue. You can put in an electric heater, but that consumes a lot of electricity and then the walls still radiate cold. I don’t find that particularly cozy for sports or music.
I don’t know much about drums and electric guitars, pianos and string instruments like a regulated climate. And even in sports, you should work up a sweat; as mentioned, controlled residential ventilation in the basement is not cheaper.