Floor plan: New construction on existing bungalow basement, 1.5-story

35 years is not really "old" even for a prefab house, but especially when it comes to insulation, it is no longer state of the art (e.g. exterior walls 18 cm thick). Since we also want to adapt the heating technology (underfloor heating) and the floor plan according to our needs, we would have to completely gut the house.

I am well aware of that; I have been involved in self-build planning for four decades. The upgrading (bringing WDIS/WDVS up to date, possibly inserting an installation level) misses the current industrial product from the manufacturer by such a margin that a conversation with a regional timber construction company (carpenter) is at least a worthwhile alternative. Do not underestimate the insulation of that time; the values were several octaves better than those of contemporary masonry houses. In the mid-80s, we are talking about mostly 30 cm hollow blocks / 36.5 cm bricks, which were decidedly outperformed by 16 cm wood frame panels with what one would probably call WDIS today.

Demolishing and disposing of a house because of underfloor heating, exactly my humor ;) The nice thing about timber frame houses is that they can be excellently converted, expanded, and renovated. With a bit of luck, you could even benefit from the new funding for [Altbausanierungen]. Underfloor heating can also be retrofitted by milling it in, for example.

Replacing a Streif house from 1987 with a new Danwood would remain qualitatively "without gain" - that would be a floor plan redesign for a hefty amount of money, which apparently doesn't play a role with you guys ???

I also find it interesting that Weberhaus and Danwood, two prefabricated builders, are at opposite ends of the price scale on the list. The plan doesn't seem to be fully thought through yet. I would also like to point out at this point that my future neighbor two houses down has been waiting for his Danwood house for about 14 months with a targeted installation date at the end of February. Even though the backlog of orders decreased somewhat last year, refurbishment is certainly faster. For an area heating system, in addition to underfloor heating, ceiling or wall heating is also an option.

It didn’t seem sensible to us to "only" save 50,000€ (number pulled out of thin air) to then plan, build, and ultimately live for decades with possibly too many compromises.

Have you ever calculated a refurbishment? Just the 30,000€ for the demolition alone already brings you a photovoltaic system with storage. With 100-150k you should get very far; then you are rather saving in the tenfold magnitude.

I also find it interesting that Weberhaus and Danwood, two prefabricated house builders, are at opposite ends of the price scale on the list. The plan doesn't seem to be fully thought through yet.

I have seen one and I also think the other, but I do not link them as "causally".

I would also like to point out here that my future neighbor two houses down has been waiting for his Danwood house for about 14 months, with a targeted installation date at the end of February.

Februaries keep coming back, perhaps in 2023 again with the usual Carnival (?)

Yes, it's about February 23, although it might also be March. By the way, here in the south, it’s not called Karneval but Fasnet ([hochdeusch Fasnacht]) ;). The exact date is not important, I just wanted to point out to the OP how long such a prefab house can take until the shell is built. And then there is still a lot of work to be done inside.

The great thing about timber frame houses is that they can be excellently converted, extended, and renovated.

I think so too. You can actually “quickly” or easily improve the insulation and install underfloor heating. Technology can also be easily replaced.

It didn’t seem sensible to us to “only” save 50K€ (number pulled out of thin air),

But you’re also interested in subsidies. That would be subsidized as well. New construction probably not, you know. Moreover, house builders should also consider how to save material nowadays, which of course also applies to disposal. The renovation subsidies exactly reflect THAT.

With a bit of luck, you could even benefit from the new subsidies for old building renovations.

Nowadays, prefabricated houses can be given a completely different look. I would talk to an architect about it, or also those from Streif Haus. A house from ’87 is really still too young for a house, unless it’s full of formaldehyde or something like that. Think it over again.