Build a single-family house as KfW55 or Efficiency House 70?

The subsidy for KfW55 is relatively low and as such not a particularly desirable goal. The question is how good or bad the insulation is in the standard without KfW already, and what payback period is acceptable for you.

If you find out the H't value for both variants, you can calculate with it and roughly estimate the amortization. The value describes the insulation quality without influence from technology or other cosmetic adjustments. Alternatively, look at which components are exactly improved and what the insulation values are for each.

Photovoltaics usually always pay off; the covered area should be as large as possible to increase the profit. Homebuilders are not necessarily the best partners for photovoltaic systems; I would plan and install that separately. Provide an empty conduit to the roof. Installing an only just sufficiently sized system for the sake of a subsidy would be a mistake.

How do you come up with these additional costs?! In 2016, we had about €4,500 extra for the whole house going from KfW70 to KfW55 (a bit more insulation and better insulation under the floor slab). Plus, subtracting the €5,000 subsidy, so a good calculation.

Our basement is almost entirely underground (rock), and only the hallway/entrance has underfloor heating, but all rooms have controlled residential ventilation. However, the values were so good that KfW55 was easily achieved.

The subsidy for KfW55 is relatively low and as such not a particularly desirable goal. The question is how good or bad the insulation in the standard without KfW already is and what payback period is acceptable for you.

If you find out the H't value for both variants, you can calculate with it and roughly estimate the payback period. The value describes the insulation quality without influence from technology or other accounting tricks. Alternatively, look at which components are exactly improved and what the insulation values are for each.

According to the advisor, the insulation in an energy-efficient house would just be the brickwork and then the plaster. Nothing else would be insulated at all, and since all the insulation material could be saved here, we would save about €20,000. It wasn't an option for us anyway, so we will stick with KfW55, but she ranted so extremely against KfW55 and also against the air-water heat pump that it seemed strange to us.

I wouldn't know how to save 20k€ with that. The woman would seem suspicious to me.