ᐅ Prefabricated House or Solid Construction House BW

Hello everyone

Here are some details about our house construction project.

Development plan/restrictions
Plot size: 591 sqm (6359 sq ft)
Slope: Yes, slight, about 1-1.5 m (3-5 ft) within the building area
Floor area: 150-160 sqm (1615-1722 sq ft)
Building envelope: approx. 13 x 19 m (43 x 62 ft)
Basement: Yes
Granny flat: Yes (approx. 50 sqm / 538 sq ft)
Number of floors: 2 full floors or knee wall minimum 160 cm (63 inches)
Roof type: Gable roof (approx. 15-25°)
Architectural style: not specified
Orientation: Eaves-side (ridge runs southeast/northwest)

Additional requirements:
Number of people and ages: Currently 2 people + 2 planned children
Room needs:
Ground floor: living/dining room, kitchen, entrance area, shower bath, guest room/office
Upper floor: 2 children’s rooms, large bathroom, master bedroom - attic
Office use: file storage and guest sleeping
Guests per year: approx. 10-20
Open or closed architecture: open
Conservative or modern construction: modern
Semi-closed kitchen: with sliding door to close
Number of dining seats: 8
Fireplace: yes
Music/sound system wall: no
Balcony, roof terrace: no
Garage, carport: yes, both

Personal budget limit for plot, house, and basement including fittings, additional building costs, and materials for DIY work (everything included): 450,000 €

We have currently held discussions with four prefab house manufacturers and are now waiting for their offers. However, a few questions arose when we roughly communicated some concerns from these manufacturers.

One of the main questions we have now is whether to build with a prefab house manufacturer or a conventional (solid) house builder/developer.

The next question is whether it makes sense, given the current interest rates, to pay extra for technology and insulation for a KfW55 house standard. (Clearly, with prefab houses this question hardly arises, but since prefab prices are currently increasing sharply, we wonder whether a conventional solid build without extensive KfW requirements might be cheaper or more efficient.)

Resulting from this is the question of technology, meaning heating, ventilation, etc.
Would it be sufficient to use a gas condensing boiler and solar panels without a ventilation system (manually opening windows is also an option), or is a heat pump (air-to-water) plus controlled ventilation and maybe photovoltaics better?
Everyone seems to say something different. One says you can’t achieve KfW55 without an air-to-water heat pump. Another says it’s possible even with gas because they have the best insulation on the market.
Another suggests adding photovoltaics with ventilation and an air-to-water heat pump... all in all, we’re quite confused about what really makes sense.

As mentioned, we will also talk to developers who build traditionally, brick by brick, and possibly consult an architect to see what results from that.

I will upload some floor plan sketches we made ourselves in the next few days, although honestly the basic questions about construction method and technology are more important to me.

Regards from Baden-Württemberg